refactor: consolidate duplicate path-finding utilities (bd-74w1, bd-4nqq)

- Add git.GetRepoRoot() with Windows path normalization
- Update beads.findGitRoot() to delegate to git.GetRepoRoot()
- Replace findBeadsDir() with beads.FindBeadsDir() across 8 files
- Remove duplicate findBeadsDir() and findGitRoot() function definitions
- Remove dead test code (TestInfoCommand, TestInfoWithNoDaemon)
- Update tests to work with consolidated APIs

Part of Code Health Review Dec 2025 epic (bd-tggf).

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

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

cmd/bd/migrate_tombstones.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"github.com/spf13/cobra"
+	"github.com/steveyegge/beads/internal/beads"
 	"github.com/steveyegge/beads/internal/types"
 	"github.com/steveyegge/beads/internal/ui"
 )
@@ -102,7 +103,7 @@ Examples:
 		}
 
 		// Find .beads directory
-		beadsDir := findBeadsDir()
+		beadsDir := beads.FindBeadsDir()
 		if beadsDir == "" {
 			if jsonOutput {
 				outputJSON(map[string]interface{}{