fix: validate BEADS_DIR env var for project files (bd-420)

Extends the fix from PR #424 to also validate the BEADS_DIR environment
variable. Previously, only the directory tree search was validated, but
BEADS_DIR could still point to a daemon-only directory.

Changes:
- Add validation to BEADS_DIR path using hasBeadsProjectFiles()
- Add comprehensive test for BEADS_DIR validation
- Test verifies both rejection of daemon-only dirs and acceptance of valid dirs

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

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

internal/beads/beads.go

@@ -275,7 +275,10 @@ func FindBeadsDir() string {
 	if beadsDir := os.Getenv("BEADS_DIR"); beadsDir != "" {
 		absBeadsDir := utils.CanonicalizePath(beadsDir)
 		if info, err := os.Stat(absBeadsDir); err == nil && info.IsDir() {
-			return absBeadsDir
+			// Validate directory contains actual project files (bd-420)
+			if hasBeadsProjectFiles(absBeadsDir) {
+				return absBeadsDir
+			}
 		}
 	}