feat(reset): implement bd reset CLI command with unit tests

Implements the bd reset command for GitHub issue #479:
- CLI command with flags: --hard, --force, --backup, --dry-run, --skip-init, --verbose
- Impact summary showing issues/tombstones to be deleted
- Confirmation prompt (skippable with --force)
- Colored output for better UX
- Unit tests for reset.go and git.go
- Fix: use --force flag in git rm to handle staged files

Part of epic bd-aydr.

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

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

internal/reset/git.go

@@ -70,8 +70,9 @@ func GitRemoveBeads(beadsDir string) error {
 	}
 
 	// Try to remove each file (git rm ignores non-existent files with --ignore-unmatch)
+	// Use --force to handle files with staged changes
 	for _, file := range jsonlFiles {
-		cmd := exec.Command("git", "rm", "--ignore-unmatch", "--quiet", file)
+		cmd := exec.Command("git", "rm", "--ignore-unmatch", "--quiet", "--force", file)
 		var stderr bytes.Buffer
 		cmd.Stderr = &stderr