[codex] Document scene stub behavior

[fdatoo]

Summary

Refs F-148.

• Add a regression test that StubSceneApplier warn-logs scene engine not yet implemented and appends a scene_applied event with slug and correlation ID.
• Update automation, Starlark, scene, and event docs so they describe the current stub behavior instead of implying a shipped Scene engine.
• Leave the stub in place; the real Scene engine remains deferred to the Scene engine spec work.

Validation

• go test ./internal/automation/action ./internal/automation ./internal/starlark ./internal/api
• go test ./...
• git diff --check

[fdatoo]

Added commit 3f99a17 for the main race CI failure.

Root cause from main run https://github.com/fdatoo/switchyard/actions/runs/25624160779: go test -race ./... timed out in internal/eventstore, with Store.Close waiting on the tailer goroutine while the tailer was blocked in sync.Cond.Wait. Close could cancel and broadcast outside the condition lock, allowing a lost wake-up.

Fixes included:

• Cancel + broadcast under the eventstore condition lock, and broadcast append notifications while holding the same lock.
• Add TestClose_WakesIdleTailer.
• Stabilize an MCP resource race-mode test whose previous coalescing assertion depended on scheduler/client speed.

Local validation:

• go test -race ./internal/eventstore -run 'TestSubscribe_DurableRequiresName|TestClose_WakesIdleTailer' -count=100 -timeout=60s
• go test -race ./internal/eventstore -count=1 -timeout=2m
• go test -race ./internal/mcp/resources -run TestEntityWatch_DeliversBurstUpdates -count=100 -timeout=60s
• go test -race ./...
• git diff --check