Adds workspace/event_log.py with an in-memory EventLog backend and a
disabled no-op variant, plus EventLogConfig nested in
ObservabilityConfig (backend / ttl_seconds / max_entries).
The event log is the append-and-query buffer that the canvas Activity
tab and platform `/activity` endpoint will read in PR-3 of the #119
stack. Two backends ship in this PR:
- InMemoryEventLog: bounded ring buffer with TTL eviction, monotonic
ids that survive eviction so cursors don't break, thread-safe for
concurrent appends from heartbeat + main loop + A2A executor.
- DisabledEventLog: no-op for `backend: disabled` — opts the
workspace out without crashing callers that propagate event ids.
Schema-only PR — no consumers wired yet. Wiring lands in PR-3.
Test coverage:
- 34 new test_event_log.py tests (100% line coverage on event_log.py)
- 9 new test_config.py tests for EventLogConfig parsing
- Concurrency stress with 8 threads × 200 appends — verifies unique
monotonic ids under contention
- TTL + max_entries eviction with injected clock (no time.sleep)
- Disabled backend contract pinned
Closes#207.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
