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/queries.sql.go

@@ -4505,6 +4505,19 @@ func (q *sqlQuerier) BackoffChatDiffStatus(ctx context.Context, arg BackoffChatD
 	return err
 }
 
+const clearChatMessageProviderResponseIDsByChatID = `-- name: ClearChatMessageProviderResponseIDsByChatID :exec
+UPDATE chat_messages
+SET provider_response_id = NULL
+WHERE chat_id = $1::uuid
+    AND deleted = false
+    AND provider_response_id IS NOT NULL
+`
+
+func (q *sqlQuerier) ClearChatMessageProviderResponseIDsByChatID(ctx context.Context, chatID uuid.UUID) error {
+	_, err := q.db.ExecContext(ctx, clearChatMessageProviderResponseIDsByChatID, chatID)
+	return err
+}
+
 const countEnabledModelsWithoutPricing = `-- name: CountEnabledModelsWithoutPricing :one
 SELECT COUNT(*)::bigint AS count
 FROM chat_model_configs
@@ -4603,6 +4616,66 @@ func (q *sqlQuerier) DeleteOldChats(ctx context.Context, arg DeleteOldChatsParam
 	return result.RowsAffected()
 }
 
+const getActiveChatsByAgentID = `-- name: GetActiveChatsByAgentID :many
+SELECT id, owner_id, workspace_id, title, status, worker_id, started_at, heartbeat_at, created_at, updated_at, parent_chat_id, root_chat_id, last_model_config_id, archived, last_error, mode, mcp_server_ids, labels, build_id, agent_id, pin_order, last_read_message_id, last_injected_context, dynamic_tools
+FROM chats
+WHERE agent_id = $1::uuid
+    AND archived = false
+    -- Active statuses only: waiting, pending, running, paused,
+    -- requires_action.
+    -- Excludes completed and error (terminal states).
+    AND status IN ('waiting', 'running', 'paused', 'pending', 'requires_action')
+ORDER BY updated_at DESC
+`
+
+func (q *sqlQuerier) GetActiveChatsByAgentID(ctx context.Context, agentID uuid.UUID) ([]Chat, error) {
+	rows, err := q.db.QueryContext(ctx, getActiveChatsByAgentID, agentID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []Chat
+	for rows.Next() {
+		var i Chat
+		if err := rows.Scan(
+			&i.ID,
+			&i.OwnerID,
+			&i.WorkspaceID,
+			&i.Title,
+			&i.Status,
+			&i.WorkerID,
+			&i.StartedAt,
+			&i.HeartbeatAt,
+			&i.CreatedAt,
+			&i.UpdatedAt,
+			&i.ParentChatID,
+			&i.RootChatID,
+			&i.LastModelConfigID,
+			&i.Archived,
+			&i.LastError,
+			&i.Mode,
+			pq.Array(&i.MCPServerIDs),
+			&i.Labels,
+			&i.BuildID,
+			&i.AgentID,
+			&i.PinOrder,
+			&i.LastReadMessageID,
+			&i.LastInjectedContext,
+			&i.DynamicTools,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Close(); err != nil {
+		return nil, err
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const getChatByID = `-- name: GetChatByID :one
 SELECT
     id, owner_id, workspace_id, title, status, worker_id, started_at, heartbeat_at, created_at, updated_at, parent_chat_id, root_chat_id, last_model_config_id, archived, last_error, mode, mcp_server_ids, labels, build_id, agent_id, pin_order, last_read_message_id, last_injected_context, dynamic_tools
@@ -6706,6 +6779,18 @@ func (q *sqlQuerier) SoftDeleteChatMessagesAfterID(ctx context.Context, arg Soft
 	return err
 }
 
+const softDeleteContextFileMessages = `-- name: SoftDeleteContextFileMessages :exec
+UPDATE chat_messages SET deleted = true
+WHERE chat_id = $1::uuid
+    AND deleted = false
+    AND content::jsonb @> '[{"type": "context-file"}]'
+`
+
+func (q *sqlQuerier) SoftDeleteContextFileMessages(ctx context.Context, chatID uuid.UUID) error {
+	_, err := q.db.ExecContext(ctx, softDeleteContextFileMessages, chatID)
+	return err
+}
+
 const unarchiveChatByID = `-- name: UnarchiveChatByID :many
 WITH chats AS (
     UPDATE chats SET