fix: make TestInterruptAutoPromotionIgnoresLaterUsageLimitIncrease deterministic (#23279)

Eliminates the timing flake in
`TestInterruptAutoPromotionIgnoresLaterUsageLimitIncrease` by making the
chatd worker loop clock-controllable.

## Changes

**`coderd/chatd/chatd.go`**
- Replace `time.NewTicker` calls in `Server.start()` with
`p.clock.NewTicker` using named quartz tags `("chatd", "acquire")` and
`("chatd", "stale-recovery")`.

**`coderd/chatd/chatd_test.go`**
- Inject `quartz.NewMock(t)` into the test via `newActiveTestServer`
config override.
- Trap the acquire ticker so the test controls exactly when pending
chats are reacquired.
- Rewrite the test flow as explicit clock-advance steps instead of
wall-clock polling.

**`AGENTS.md`**
- Document the PR title scope rule (scope must be a real path containing
all changed files).

## Validation
- `go test ./coderd/chatd -run
TestInterruptAutoPromotionIgnoresLaterUsageLimitIncrease -count=100` ✅
- `go test ./coderd/chatd` ✅
- `make lint` ✅

AGENTS.md

@@ -208,6 +208,11 @@ seems like it should use `time.Sleep`, read through https://github.com/coder/qua
 
 - Follow [Uber Go Style Guide](https://github.com/uber-go/guide/blob/master/style.md)
 - Commit format: `type(scope): message`
+- PR titles follow the same `type(scope): message` format.
+- When you use a scope, it must be a real filesystem path containing every
+  changed file.
+- Use a broader path scope, or omit the scope, for cross-cutting changes.
+- Example: `fix(coderd/chatd): ...` for changes only in `coderd/chatd/`.
 
 ### Frontend Patterns