Improve agent-facing history endpoints (Phase 1)

- Fix /api/history/{type} to search the full persistent store instead of
  only the bounded in-memory ring buffer, so filtered history isn't
  silently truncated.
- Add structured type/since/until filters to /api/history/search and
  /api/history/wait, alongside the existing free-text query.
- Add GET /api/traces/{traceId} to fetch all entries for a trace in one
  call instead of relying on substring search.
- Rate-limit /api/history/wait per client IP.
- Add docs/PLAN.md tracking the phased improvement plan; update README
  and agents_example.md for the new/changed endpoints.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -95,6 +95,16 @@ gRPC services for traces, metrics, and logs using standard OTLP protobuf definit
 - `GET /api/history/traces` - Retrieve only trace history
 - `GET /api/history/metrics` - Retrieve only metrics history
 - `GET /api/history/logs` - Retrieve only logs history
+- `GET /api/history/search` - Search history. Supports `q` (free-text
+  substring), `type`, `since`/`until` (unix nanoseconds), and `limit`.
+- `GET /api/history/wait` - Block until a matching entry arrives. Same
+  filters as `search`, plus `timeout` (seconds, default 30). Rate-limited
+  per client IP.
+- `GET /api/traces/{traceId}` - Retrieve all stored entries (spans and
+  correlated logs) referencing a given trace id.
+
+All endpoints above search the full persistent store (when `OTELLOG_PATH`
+storage is enabled), not just the in-memory ring buffer.
 
 ### Web Interface