feat: add silent quick-capture mode (bd q)

Add a truly silent quick-capture command that outputs only the issue ID,
enabling seamless script integration and reducing noise in AI conversations.

Changes:
- Add --silent flag to 'bd create' command
- Create new 'bd q' alias for quick capture
- Suppress warnings in silent/quiet mode

Usage:
  bd q "Fix login bug"           # Outputs only: bd-a1b2
  ISSUE=$(bd q "New feature")    # Capture ID in variable
  bd create "Task" --silent       # Same as bd q

Closes #540

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.beads/.gitignore

@@ -30,3 +30,5 @@ beads.right.meta.json
 !issues.jsonl
 !metadata.json
 !config.json
+deletions.jsonl
+deletions.jsonl.migrated