chore: remove agents experiment flag and mark feature as beta (#24432)

Remove the `ExperimentAgents` feature flag so the Agents feature is
always available without requiring `--experiments=agents`. The feature
is now in beta.

Existing deployments that still pass `--experiments=agents` will get a
harmless "ignoring unknown experiment" warning on startup.

### Changes

**Backend:**
- Remove `RequireExperimentWithDevBypass` middleware from chat and MCP
server routes
- Always include `AgentsAccessRole` in assignable site roles (later
refactored to org-scoped on main; rebase keeps that)
- Always set `AgentsTabVisible = true`, then drop the entire dead
`AgentsTabVisible` metadata pipeline (Go htmlState field,
populateHTMLState goroutine, HTML meta tag, useEmbeddedMetadata
registration, mock); no production consumer reads it. `AgentsNavItem`
already gates on `permissions.createChat`.
- Make `blob:` CSP `img-src` addition unconditional
- Remove `ExperimentAgents` constant, `DisplayName` case, and
`ExperimentsKnown` entry

**CLI:**
- Graduate the agents TUI from `coder exp agents` to `coder agents`
(moved from `AGPLExperimental()` to `CoreSubcommands()`)
- Drop the `agent` alias so it does not collide with the hidden
workspace-agent command
- Rename implementation files `cli/exp_agents_*.go` -> `cli/agents_*.go`
and internal identifiers (`expChatsTUIModel` -> `chatsTUIModel`,
`newExpChatsTUIModel` -> `newChatsTUIModel`, `setupExpAgentsBackend` ->
`setupAgentsBackend`, `startExpAgentsSession` -> `startAgentsSession`,
`expAgentsPtr` -> `agentsPtr`, `expAgentsSession` -> `agentsSession`,
`TestExpAgents*` -> `TestAgents*`). `expClient` (the
`*codersdk.ExperimentalClient` local) is kept; `coderd/exp_chats*.go`
and other still-experimental `cli/exp_*.go` commands are intentionally
untouched.

**Frontend:**
- Remove experiment check from `AgentsNavItem` - render when
`canCreateChat` is true
- Remove `agentsEnabled` experiment check from `WorkspacesPage`, then
gate `chatsByWorkspace` on `permissions.createChat` so users without
chat access don't trigger the per-page DB query (Copilot review
feedback)
- Add `FeatureStageBadge` (beta) next to the Coder logo in the Agents
sidebar (desktop + mobile)

**Docs:**
- Remove experiment flag setup instructions from `early-access.md` and
`getting-started.md` (and rename `early-access.md`'s "Enable Coder
Agents" heading to "Set up Coder Agents", since there is no enablement
step left)
- Update `chats-api.md` and `getting-started.md`'s Chats API note to say
"beta" instead of "experimental"
- `docs/manifest.json`: drop "experimental" from the Chats API sidebar
description
- `make gen` regenerated `docs/reference/cli/agents.md` and the CLI
index
- `scripts/check_emdash.sh`: exclude `cli/testdata/*.golden` and
`enterprise/cli/testdata/*.golden` from the new repo-wide emdash lint,
since serpent emits emdash borders in every generated `--help` golden
file

**Tests:**
- Remove `ExperimentAgents` setup from all test files (14 occurrences
across 7 files)
- Update stale "with the agents experiment" comments in
`coderd/x/chatd/integration_test.go` and `coderd/mcp_test.go`


<img width="1185" height="900" alt="image"
src="https://github.com/user-attachments/assets/b420bc8f-41d6-42c6-abd8-ad572533d651"
/>


> 🤖 Generated by Coder Agents

docs/ai-coder/agents/chats-api.md

@@ -1,7 +1,7 @@
 # Chats API
 
 > [!NOTE]
-> The Chats API is experimental and gated behind the `agents` experiment flag.
+> The Chats API is in beta.
 > Endpoints live under `/api/experimental/chats` and may change without notice.
 
 The Chats API lets you create and interact with Coder Agents

docs/ai-coder/agents/early-access.md

@@ -40,27 +40,11 @@ Functionality available during Early Access may be a subset of planned
 capabilities. Some features may be incomplete, experimental, or subject to
 redesign.
 
-## Enable Coder Agents
+## Set up Coder Agents
 
-Coder Agents is experimental and must not be deployed to production
-environments. It is gated behind the `agents` experiment flag. To enable it,
-pass the flag when starting the Coder server using an environment variable
-or CLI flag:
+Coder Agents is available by default. No experiment flags are required.
 
-```sh
-CODER_EXPERIMENTS="agents" coder server
-# or
-coder server --experiments=agents
-```
-
-If you are already using other experiments, add `agents` to the
-comma-separated list:
-
-```sh
-CODER_EXPERIMENTS="agents,oauth2,mcp-server-http" coder server
-```
-
-Once the server restarts with the experiment enabled:
+To get started:
 
 1. Navigate to the **Agents** page in the Coder dashboard.
 1. Open **Admin** settings and configure at least one LLM provider and model.

docs/ai-coder/agents/getting-started.md

@@ -1,6 +1,6 @@
 # Getting Started
 
-This guide walks platform teams and administrators through enabling Coder
+This guide walks platform teams and administrators through setting up Coder
 Agents, preparing your deployment, and running your first Coder Agent.
 
 > [!NOTE]
@@ -12,8 +12,7 @@ Agents, preparing your deployment, and running your first Coder Agent.
 
 Before you begin, confirm the following:
 
-- **Coder deployment** running the latest release with the `agents`
-  experiment flag available.
+- **Coder deployment** running the latest release.
 - **LLM provider credentials** — an API key for at least one
   [supported provider](./models.md) (Anthropic, OpenAI, Google, Azure OpenAI,
   AWS Bedrock, OpenAI Compatible, OpenRouter, or Vercel AI Gateway).
@@ -22,40 +21,19 @@ Before you begin, confirm the following:
 - **At least one template** with a
   [descriptive name and description](./platform-controls/template-optimization.md)
   for the agent to select when provisioning workspaces.
-- **Admin access** to the Coder deployment for enabling the experiment and
-  configuring providers.
+- **Admin access** to the Coder deployment for configuring providers.
 - **Coder Agents User role** assigned to each user who needs to interact with Coder Agents.
   Owners can assign this from **Admin** > **Users**. See
-  [Grant Coder Agents User](#step-3-grant-coder-agents-user) below.
+  [Grant Coder Agents User](#step-2-grant-coder-agents-user) below.
 
-## Step 1: Enable the experiment
-
-Coder Agents is gated behind the `agents` experiment flag. Pass it when
-starting the Coder server:
-
-```sh
-CODER_EXPERIMENTS="agents" coder server
-# or
-coder server --experiments=agents
-```
-
-If you already use other experiments, add `agents` to the comma-separated list:
-
-```sh
-CODER_EXPERIMENTS="agents,oauth2,mcp-server-http" coder server
-```
-
-See [Enable Coder Agents](./early-access.md#enable-coder-agents) for full
-details.
-
-## Step 2: Configure an LLM provider and model
+## Step 1: Configure an LLM provider and model
 
 > [!IMPORTANT]
 > Configuring providers, models, and system prompts requires the
 > **Owner** role (Coder administrator). Non-admin users cannot access the
 > Admin panel or modify deployment-level Agents configuration.
 
-Once the server restarts with the experiment enabled:
+To configure Coder Agents:
 
 1. Navigate to the **Agents** page in the Coder dashboard.
 1. Click **Admin** to open the configuration dialog.
@@ -72,7 +50,7 @@ Detailed instructions for each provider and model option are in the
 > Start with a single frontier model to validate your setup before adding
 > additional providers.
 
-## Step 3: Grant Coder Agents User
+## Step 2: Grant Coder Agents User
 
 The **Coder Agents User** role controls which users can interact with Coder Agents.
 Members do not have Coder Agents User by default.
@@ -105,7 +83,7 @@ coder users list -o json \
     done
 ```
 
-## Step 4: Start your first Coder Agent
+## Step 3: Start your first Coder Agent
 
 1. Go to the **Agents** page in the Coder dashboard.
 1. Select a model from the dropdown (your default will be pre-selected).
@@ -266,7 +244,7 @@ rather than developer session tokens. Keep automation credentials
 narrowly scoped.
 
 > [!NOTE]
-> The Chats API is experimental and may change without notice.
+> The Chats API is in beta and may change without notice.
 > See [Chats API](./chats-api.md) for the full endpoint reference.
 
 ### Add workspace context with AGENTS.md