docs: Add event CLI documentation to README

Document the new event command and its subcommands:
- event list: List events with optional filtering
- event add: Add events to MCAP files (appends in place)
- event delete: Remove events from MCAP files
- event clip: Extract MCAP portion around an event timestamp

Author: claude

README.md

@@ -276,3 +276,51 @@ pybag inspect channels recording.mcap --topic "/camera/*"
 pybag inspect metadata recording.mcap --name "calibration"
 pybag inspect attachments recording.mcap --name "calib.yaml" --data
 ```
+
+### event
+
+Manage events in MCAP files. Events are markers that indicate when something
+significant happens in the recording (e.g., collisions, waypoints, incidents).
+Events are stored as metadata records.
+
+**List events:**
+
+```bash
+pybag event list recording.mcap
+pybag event list recording.mcap --name "collision"
+pybag event list recording.mcap --json
+```
+
+**Add events (appends in place):**
+
+```bash
+pybag event add recording.mcap "collision" 10.5
+pybag event add recording.mcap "start" 0.0 --description "Recording started"
+pybag event add recording.mcap "waypoint" 25.0 --extra waypoint_id=42
+```
+
+**Delete events:**
+
+```bash
+pybag event delete recording.mcap -o cleaned.mcap
+pybag event delete recording.mcap --name "collision" -o cleaned.mcap
+pybag event delete recording.mcap --start-time 5.0 --end-time 10.0 -o cleaned.mcap
+```
+
+**Clip around an event:**
+
+Extract a portion of the recording centered on an event's timestamp:
+
+```bash
+# Clip 5s before and 10s after the event
+pybag event clip recording.mcap "collision" --before 5 --after 10
+
+# Symmetric: 3s before AND after (only specify one)
+pybag event clip recording.mcap "incident" --before 3
+
+# Default: 5s before and after
+pybag event clip recording.mcap "start"
+
+# With topic filtering
+pybag event clip recording.mcap "crash" --include-topic "/camera/*" -o clip.mcap
+```