refactor: remove unused bd pin/unpin/hook commands (bd-x0zl)

Analysis found these commands are dead code:
- gt never calls `bd pin` - uses `bd update --status=pinned` instead
- Beads.Pin() wrapper exists but is never called
- bd hook functionality duplicated by gt mol status
- Code comment says "pinned field is cosmetic for bd hook visibility"

Removed:
- cmd/bd/pin.go
- cmd/bd/unpin.go
- cmd/bd/hook.go

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

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

cmd/bd/doctor/fix/common.go

@@ -12,6 +12,13 @@ import (
 // This prevents fork bombs when tests call functions that execute bd subcommands.
 var ErrTestBinary = fmt.Errorf("running as test binary - cannot execute bd subcommands")
 
+func newBdCmd(bdBinary string, args ...string) *exec.Cmd {
+	fullArgs := append([]string{"--no-daemon"}, args...)
+	cmd := exec.Command(bdBinary, fullArgs...) // #nosec G204 -- bdBinary from validated executable path
+	cmd.Env = append(os.Environ(), "BEADS_NO_DAEMON=1")
+	return cmd
+}
+
 // getBdBinary returns the path to the bd binary to use for fix operations.
 // It prefers the current executable to avoid command injection attacks.
 // Returns ErrTestBinary if running as a test binary to prevent fork bombs.