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>

go.mod

@@ -5,6 +5,7 @@ go 1.24.0
 toolchain go1.24.11
 
 require (
+	github.com/BurntSushi/toml v1.6.0
 	github.com/anthropics/anthropic-sdk-go v1.19.0
 	github.com/charmbracelet/huh v0.8.0
 	github.com/charmbracelet/lipgloss v1.1.0