feat: add bd resolve-conflicts command for JSONL merge conflicts (bd-7e7ddffa.1)

Implements a new command to resolve git merge conflict markers in JSONL files.

Features:
- Mechanical mode (default): deterministic merge using updated_at timestamps
- Closed status wins over open
- Higher priority (lower number) wins
- Notes are concatenated when different
- Dependencies are unioned
- Dry-run mode to preview changes
- JSON output for agent integration
- Automatic backup creation before changes

The command defaults to resolving .beads/beads.jsonl but accepts any file path.

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

cmd/bd/main.go

@@ -310,6 +310,7 @@ var rootCmd = &cobra.Command{
 			"prime",
 			"quickstart",
 			"repair",
+			"resolve-conflicts",
 			"setup",
 			"version",
 			"zsh",