feat: add CLI commands for managing chat context from workspaces (#24105)

Adds `coder exp chat context add` and `coder exp chat context clear`
commands that run inside a workspace to manage chat context files via
the agent token.

`add` reads instruction and skill files from a directory (defaulting to
cwd) and inserts them as context-file messages into an active chat.
Multiple calls are additive — `instructionFromContextFiles` already
accumulates all context-file parts across messages.

`clear` soft-deletes all context-file messages, causing
`contextFileAgentID()` to return `!found` on the next turn, which
triggers `needsInstructionPersist=true` and re-fetches defaults from the
agent.

Both commands auto-detect the target chat via `CODER_CHAT_ID` (already
set by `agentproc` on chat-spawned processes), or fall back to
single-active-chat resolution for the agent. The `--chat` flag overrides
both.

Also adds sub-agent context inheritance: `createChildSubagentChat` now
copies parent context-file messages to child chats at spawn time, so
delegated sub-agents share the same instruction context without
independently re-fetching from the workspace agent.

<details><summary>Implementation details</summary>

**New files:**
- `cli/exp_chat.go` — CLI command tree under `coder exp chat context`

**Modified files:**
- `agent/agentcontextconfig/api.go` — `ConfigFromDir()` reads context
from an arbitrary directory without env vars
- `codersdk/agentsdk/agentsdk.go` — `AddChatContext`/`ClearChatContext`
SDK methods
- `coderd/workspaceagents.go` — POST/DELETE handlers on
`/workspaceagents/me/chat-context`
- `coderd/coderd.go` — Route registration
- `coderd/database/queries/chats.sql` — `GetActiveChatsByAgentID`,
`SoftDeleteContextFileMessages`
- `coderd/database/dbauthz/dbauthz.go` — RBAC implementations for new
queries
- `coderd/x/chatd/subagent.go` — `copyParentContextFiles` for sub-agent
inheritance
- `cli/root.go` — Register `chatCommand()` in `AGPLExperimental()`

**Auth pattern:** Uses `AgentAuth` (same as `coder external-auth`) —
agent token via `CODER_AGENT_TOKEN` + `CODER_AGENT_URL` env vars.

</details>

> 🤖 Generated by Coder Agents

---------

Co-authored-by: Michael Suchacz <203725896+ibetitsmike@users.noreply.github.com>

coderd/database/dbmetrics/querymetrics.go

@@ -280,6 +280,14 @@ func (m queryMetricsStore) CleanupDeletedMCPServerIDsFromChats(ctx context.Conte
 	return r0
 }
 
+func (m queryMetricsStore) ClearChatMessageProviderResponseIDsByChatID(ctx context.Context, chatID uuid.UUID) error {
+	start := time.Now()
+	r0 := m.s.ClearChatMessageProviderResponseIDsByChatID(ctx, chatID)
+	m.queryLatencies.WithLabelValues("ClearChatMessageProviderResponseIDsByChatID").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "ClearChatMessageProviderResponseIDsByChatID").Inc()
+	return r0
+}
+
 func (m queryMetricsStore) CountAIBridgeInterceptions(ctx context.Context, arg database.CountAIBridgeInterceptionsParams) (int64, error) {
 	start := time.Now()
 	r0, r1 := m.s.CountAIBridgeInterceptions(ctx, arg)
@@ -968,6 +976,14 @@ func (m queryMetricsStore) GetActiveAISeatCount(ctx context.Context) (int64, err
 	return r0, r1
 }
 
+func (m queryMetricsStore) GetActiveChatsByAgentID(ctx context.Context, agentID uuid.UUID) ([]database.Chat, error) {
+	start := time.Now()
+	r0, r1 := m.s.GetActiveChatsByAgentID(ctx, agentID)
+	m.queryLatencies.WithLabelValues("GetActiveChatsByAgentID").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "GetActiveChatsByAgentID").Inc()
+	return r0, r1
+}
+
 func (m queryMetricsStore) GetActivePresetPrebuildSchedules(ctx context.Context) ([]database.TemplateVersionPresetPrebuildSchedule, error) {
 	start := time.Now()
 	r0, r1 := m.s.GetActivePresetPrebuildSchedules(ctx)
@@ -4104,6 +4120,14 @@ func (m queryMetricsStore) SoftDeleteChatMessagesAfterID(ctx context.Context, ar
 	return r0
 }
 
+func (m queryMetricsStore) SoftDeleteContextFileMessages(ctx context.Context, chatID uuid.UUID) error {
+	start := time.Now()
+	r0 := m.s.SoftDeleteContextFileMessages(ctx, chatID)
+	m.queryLatencies.WithLabelValues("SoftDeleteContextFileMessages").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "SoftDeleteContextFileMessages").Inc()
+	return r0
+}
+
 func (m queryMetricsStore) TryAcquireLock(ctx context.Context, pgTryAdvisoryXactLock int64) (bool, error) {
 	start := time.Now()
 	r0, r1 := m.s.TryAcquireLock(ctx, pgTryAdvisoryXactLock)