perf(chatd): fix six scale bottlenecks identified by benchmarking (#22957)

## Summary

Scale-tested the `chatd` package with mock-based benchmarks to identify
performance bottlenecks. This PR fixes 6 of the 8 identified issues,
ranked by severity.

## Changes

### 1. Parallel tool execution (HIGH) — `chatloop.go`
`executeTools` ran tool calls sequentially. Now dispatches all calls
concurrently via goroutines with `sync.WaitGroup`. Results are
pre-allocated by index (no mutex needed). `onResult` callbacks fire as
each tool completes.

### 2. Pubsub-backed subagent await (HIGH) — `subagent.go`
`awaitSubagentCompletion` polled the DB every 200ms. Now subscribes to
the child chat's `ChatStreamNotifyChannel` via pubsub for near-instant
notifications. Fallback poll reduced to 5s. Falls back to 200ms only
when `pubsub == nil` (single-instance / in-memory).

### 3. Per-chat stream locking (MEDIUM) — `chatd.go`
Replaced single global `streamMu` + `map[uuid.UUID]*chatStreamState`
with `sync.Map` where each `chatStreamState` has its own `sync.Mutex`.
Zero cross-chat contention.

### 4. Batch chat acquisition (MEDIUM) — `chatd.go`
`processOnce` acquired 1 chat per tick. Now loops up to
`maxChatsPerAcquire = 10` per tick, avoiding idle time when many chats
are pending.

### 5. Reduced heartbeat frequency (LOW-MEDIUM) — `chatd.go`
`chatHeartbeatInterval` changed from 30s to 60s. Safe given the 5-minute
`DefaultInFlightChatStaleAfter`.

### 6. O(depth) descendant check (LOW) — `subagent.go`
Replaced top-down BFS (`O(total_descendants)` queries) with bottom-up
parent-chain walk (`O(depth)` queries). Includes cycle protection.

## Not addressed (intentionally)
- Message serialization overhead
- Buffer eviction (`buffer[1:]` pattern)

coderd/database/queries/chats.sql

@@ -257,9 +257,9 @@ WHERE
 RETURNING
     *;
 
--- name: AcquireChat :one
--- Acquires a pending chat for processing. Uses SKIP LOCKED to prevent
--- multiple replicas from acquiring the same chat.
+-- name: AcquireChats :many
+-- Acquires up to @num_chats pending chats for processing. Uses SKIP LOCKED
+-- to prevent multiple replicas from acquiring the same chat.
 UPDATE
     chats
 SET
@@ -269,7 +269,7 @@ SET
     updated_at = @started_at::timestamptz,
     worker_id = @worker_id::uuid
 WHERE
-    id = (
+    id = ANY(
         SELECT
             id
         FROM
@@ -281,7 +281,7 @@ WHERE
         FOR UPDATE
             SKIP LOCKED
         LIMIT
-            1
+            @num_chats::int
     )
 RETURNING
     *;