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>

default.nix

@@ -1,14 +1,14 @@
 { pkgs, self }:
 pkgs.buildGoModule {
   pname = "beads";
-  version = "0.30.7";
+  version = "0.37.0";
 
   src = self;
 
   # Point to the main Go package
   subPackages = [ "cmd/bd" ];
   doCheck = false;
-  # Go module dependencies hash (computed via nix build)
+  # Go module dependencies hash - if build fails with hash mismatch, update with the "got:" value
   vendorHash = "sha256-Brzb6HZHYtF8LTkP3uQ21GG72c5ekzSkQ2EdrqkdeO0=";
 
   # Git is required for tests