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>

commands/comments.md

@@ -5,6 +5,8 @@ argument-hint: [issue-id]
 
 View or add comments to a beads issue.
 
+Comments are separate from issue properties (title, description, etc.) because they serve a different purpose: they're a **discussion thread** rather than **singular editable fields**. Use `bd comments` for threaded conversations and `bd edit` for core issue metadata.
+
 ## View Comments
 
 To view all comments on an issue:

commands/update.md

@@ -16,6 +16,8 @@ If arguments are missing, ask the user for:
 
 Use the beads MCP `update` tool to apply the changes. Show the updated issue to confirm the change.
 
+**Note:** Comments are managed separately with `bd comments add`. The `update` command is for singular, versioned properties (title, status, priority, etc.), while comments form a discussion thread that's appended to, not updated.
+
 Common workflows:
 - Start work: Update status to `in_progress`
 - Mark blocked: Update status to `blocked`