docs(coderd/x/chatd): correct timing, route, hook, and tool details in ARCHITECTURE.md - #29154

Corrects statements in coderd/x/chatd/ARCHITECTURE.md that disagree with the current code, and fills in the omissions found alongside them. Docs-only; each item below names the code that backs it.

• Acquisition loop: the timer fires every 1s and a lease expires after 5m, not 30s/30s. Both are chatd.New defaults (DefaultPendingChatAcquireInterval, DefaultInFlightChatStaleAfter, coderd/x/chatd/chatd.go:65-68, 3120-3123); coderd/coderd.go:937-965 sets neither and no deployment flag exposes them. The 30s values are the chatWorkerOptions fallbacks in options.go:24,30. The acquire predicate also treats a null runner_id as unowned and checks chat_heartbeats for the current (chat_id, runner_id) (coderd/database/queries/chats.sql:2480-2489).
• Heartbeat loop: the interval is 30s (DefaultChatHeartbeatInterval, chatd.go:83), not 9s, so a lease survives ten intervals after the last successful write rather than three. The cleanup loop runs every 30s and uses the same parameterized threshold (runner_manager.go:499, chats.sql:2522-2524).
• HTTP endpoints: routes are registered once and mounted under both /api/experimental and /api/v2 (coderd/chat_routes.go:33-43, coderd/coderd.go:1394, 1451). Added the experimental-only routes (chat_routes.go:62-77, 128-139, 166-179) and the reserved 404 segments on each mount (chat_routes.go:52-60, 78-93).
• message_part stream events already carry history_version and generation_attempt (codersdk/chats.go:1692-1698); the "should additionally include" note is replaced with a statement of current behavior.
• Lifecycle hooks: enumerated the seven event types (codersdk/x/agenthooks/types.go:31-37) and noted that a permission on any event other than user_prompt_submit or pre_tool_use is rejected as an invalid response (coderd/x/agenthooks/dispatch/dispatcher.go:590-599, 363-370).
• Generation goroutine: listed the subagent tools and the close_agent alias (coderd/x/chatd/subagent_catalog.go:17-18,43-50, subagent.go:41-47).
• Replaced the two TODO: notes with prose describing openAIReasoningEffort (coderd/x/chatd/chatprovider/reasoningeffort.go:183-192) and UsesResponsesAPI (coderd/x/chatd/chatopenai/transport.go:64-78, chatprovider/chatprovider.go:767-771).

Follow-ups noted during review, out of scope here because they are new content rather than corrections:

• The chat:ownership emission rule (Pubsub section) decides "no fresh heartbeat row" using a hardcoded 30s in coderd/x/chatd/chatstate/machine.go:20, distinct from the 5m lease threshold. The doc should state both thresholds.
• The concurrent agent limiter counts a chat as active while its heartbeat row is younger than the same 5m threshold (chatd.go:3102-3105, chats.sql:2975, 2992, 3007), so a crashed replica's chats hold capacity slots until the lease expires. The doc does not mention this.

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