feat: add run deletion, session reopen, and feed merging (v0.6.0)

- DELETE /api/sessions/{id}/runs/{run_id} removes stopped runs
- POST /api/sessions/{id}/reopen transitions Completed → Active
- Optional merge_feeds field on POST /api/sessions/{id}/runs combines
  frames from multiple feeds with priority-based deduplication
- In-place merge_frames() avoids 2× peak memory
- All new fields use #[serde(default)] for backward compatibility
- Updated README.md with new endpoints and feed merging docs

Cargo.lock

@@ -786,7 +786,7 @@ checksum = "b39cdef0fa800fc44525c84ccb54a029961a8215f9619753635a9c0d2538d46d"
 
 [[package]]
 name = "rusty-telemetry"
-version = "0.5.0"
+version = "0.6.0"
 dependencies = [
  "axum",
  "chrono",