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>

cli/exp_chat.go

@@ -0,0 +1,194 @@
+package cli
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/google/uuid"
+	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/v2/agent/agentcontextconfig"
+	"github.com/coder/coder/v2/codersdk/agentsdk"
+	"github.com/coder/serpent"
+)
+
+func (r *RootCmd) chatCommand() *serpent.Command {
+	return &serpent.Command{
+		Use:   "chat",
+		Short: "Manage agent chats",
+		Long:  "Commands for interacting with chats from within a workspace.",
+		Handler: func(i *serpent.Invocation) error {
+			return i.Command.HelpHandler(i)
+		},
+		Children: []*serpent.Command{
+			r.chatContextCommand(),
+		},
+	}
+}
+
+func (r *RootCmd) chatContextCommand() *serpent.Command {
+	return &serpent.Command{
+		Use:   "context",
+		Short: "Manage chat context",
+		Long:  "Add or clear context files and skills for an active chat session.",
+		Handler: func(i *serpent.Invocation) error {
+			return i.Command.HelpHandler(i)
+		},
+		Children: []*serpent.Command{
+			r.chatContextAddCommand(),
+			r.chatContextClearCommand(),
+		},
+	}
+}
+
+func (*RootCmd) chatContextAddCommand() *serpent.Command {
+	var (
+		dir    string
+		chatID string
+	)
+	agentAuth := &AgentAuth{}
+	cmd := &serpent.Command{
+		Use:   "add",
+		Short: "Add context to an active chat",
+		Long: "Read instruction files and discover skills from a directory, then add " +
+			"them as context to an active chat session. Multiple calls " +
+			"are additive.",
+		Handler: func(inv *serpent.Invocation) error {
+			ctx := inv.Context()
+			ctx, stop := inv.SignalNotifyContext(ctx, StopSignals...)
+			defer stop()
+
+			if dir == "" && inv.Environ.Get("CODER") != "true" {
+				return xerrors.New("this command must be run inside a Coder workspace (set --dir to override)")
+			}
+
+			client, err := agentAuth.CreateClient()
+			if err != nil {
+				return xerrors.Errorf("create agent client: %w", err)
+			}
+
+			resolvedDir := dir
+			if resolvedDir == "" {
+				resolvedDir, err = os.Getwd()
+				if err != nil {
+					return xerrors.Errorf("get working directory: %w", err)
+				}
+			}
+			resolvedDir, err = filepath.Abs(resolvedDir)
+			if err != nil {
+				return xerrors.Errorf("resolve directory: %w", err)
+			}
+			info, err := os.Stat(resolvedDir)
+			if err != nil {
+				return xerrors.Errorf("cannot read directory %q: %w", resolvedDir, err)
+			}
+			if !info.IsDir() {
+				return xerrors.Errorf("%q is not a directory", resolvedDir)
+			}
+
+			parts := agentcontextconfig.ContextPartsFromDir(resolvedDir)
+			if len(parts) == 0 {
+				_, _ = fmt.Fprintln(inv.Stderr, "No context files or skills found in "+resolvedDir)
+				return nil
+			}
+
+			// Resolve chat ID from flag or auto-detect.
+			resolvedChatID, err := parseChatID(chatID)
+			if err != nil {
+				return err
+			}
+
+			resp, err := client.AddChatContext(ctx, agentsdk.AddChatContextRequest{
+				ChatID: resolvedChatID,
+				Parts:  parts,
+			})
+			if err != nil {
+				return xerrors.Errorf("add chat context: %w", err)
+			}
+
+			_, _ = fmt.Fprintf(inv.Stdout, "Added %d context part(s) to chat %s\n", resp.Count, resp.ChatID)
+			return nil
+		},
+		Options: serpent.OptionSet{
+			{
+				Name:        "Directory",
+				Flag:        "dir",
+				Description: "Directory to read context files and skills from. Defaults to the current working directory.",
+				Value:       serpent.StringOf(&dir),
+			},
+			{
+				Name:        "Chat ID",
+				Flag:        "chat",
+				Env:         "CODER_CHAT_ID",
+				Description: "Chat ID to add context to. Auto-detected from CODER_CHAT_ID, the only active chat, or the only top-level active chat.",
+				Value:       serpent.StringOf(&chatID),
+			},
+		},
+	}
+	agentAuth.AttachOptions(cmd, false)
+	return cmd
+}
+
+func (*RootCmd) chatContextClearCommand() *serpent.Command {
+	var chatID string
+	agentAuth := &AgentAuth{}
+	cmd := &serpent.Command{
+		Use:   "clear",
+		Short: "Clear context from an active chat",
+		Long: "Soft-delete all context-file and skill messages from an active chat. " +
+			"The next turn will re-fetch default context from the agent.",
+		Handler: func(inv *serpent.Invocation) error {
+			ctx := inv.Context()
+			ctx, stop := inv.SignalNotifyContext(ctx, StopSignals...)
+			defer stop()
+
+			client, err := agentAuth.CreateClient()
+			if err != nil {
+				return xerrors.Errorf("create agent client: %w", err)
+			}
+
+			resolvedChatID, err := parseChatID(chatID)
+			if err != nil {
+				return err
+			}
+
+			resp, err := client.ClearChatContext(ctx, agentsdk.ClearChatContextRequest{
+				ChatID: resolvedChatID,
+			})
+			if err != nil {
+				return xerrors.Errorf("clear chat context: %w", err)
+			}
+
+			if resp.ChatID == uuid.Nil {
+				_, _ = fmt.Fprintln(inv.Stdout, "No active chats to clear.")
+			} else {
+				_, _ = fmt.Fprintf(inv.Stdout, "Cleared context from chat %s\n", resp.ChatID)
+			}
+			return nil
+		},
+		Options: serpent.OptionSet{{
+			Name:        "Chat ID",
+			Flag:        "chat",
+			Env:         "CODER_CHAT_ID",
+			Description: "Chat ID to clear context from. Auto-detected from CODER_CHAT_ID, the only active chat, or the only top-level active chat.",
+			Value:       serpent.StringOf(&chatID),
+		}},
+	}
+	agentAuth.AttachOptions(cmd, false)
+	return cmd
+}
+
+// parseChatID returns the chat UUID from the flag value (which
+// serpent already populates from --chat or CODER_CHAT_ID). Returns
+// uuid.Nil if empty (the server will auto-detect).
+func parseChatID(flagValue string) (uuid.UUID, error) {
+	if flagValue == "" {
+		return uuid.Nil, nil
+	}
+	parsed, err := uuid.Parse(flagValue)
+	if err != nil {
+		return uuid.Nil, xerrors.Errorf("invalid chat ID %q: %w", flagValue, err)
+	}
+	return parsed, nil
+}