Capture invalid announcement rejections in Phase 4

Update parse failures script to also extract 'Invalid announcement'
rejections from logs. These are announcement events that failed
validation (e.g., multiple clone tags instead of single tag with
multiple values).

Changes:
- Search for 'Event rejected by write policy' pattern with 'Invalid announcement'
- Search for 'Rejected repository announcement' pattern from builder
- Extract event_id, kind, and reason from rejection logs
- Combine with [PARSE_FAIL] entries in output
- Deduplicate entries by event_id
- Update header to clarify both patterns are captured
- Update migration guide to document this
- Fix SIGPIPE handling in purgatory script (minor)

This captures the ~446 unique announcements rejected for NIP-34 format
violations (multiple clone tags), which were previously unexplained
in the migration analysis.

0 files changed, 0 insertions, 0 deletions