Add 'bd clean' command to remove temporary merge artifacts

This command cleans up temporary files created during git merges by reading
patterns directly from .beads/.gitignore (Merge artifacts section).

Files removed:
- 3-way merge snapshots (beads.base.jsonl, beads.left.jsonl, beads.right.jsonl)
- Merge metadata (*.meta.json)
- Git merge driver temp files (*.json[0-9], *.jsonl[0-9])

Files preserved:
- beads.jsonl (source of truth)
- beads.db (SQLite database)
- metadata.json, config.yaml
- All daemon files

Usage:
  bd clean           # Clean up temporary files
  bd clean --dry-run # Preview what would be deleted

Implementation:
- Reads patterns from .beads/.gitignore instead of hardcoding them
- No --force flag needed - just runs by default
- Only cleans truly temporary merge artifacts, never the database

Also:
- Restored beads.jsonl to 538 issues from commit 6cd3a32
- Set issue-prefix to "bd" in config.yaml

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.beads/config.yaml

@@ -6,7 +6,7 @@
 # Issue prefix for this repository (used by bd init)
 # If not set, bd init will auto-detect from directory name
 # Example: issue-prefix: "myproject" creates issues like "myproject-1", "myproject-2", etc.
-# issue-prefix: ""
+issue-prefix: "bd"
 
 # Use no-db mode: load from JSONL, no SQLite, write back after each command
 # When true, bd will use .beads/issues.jsonl as the source of truth