feat: add prepare-commit-msg hook for agent identity trailers (bd-luso)

Automatically adds trailers to commits when running in Gas Town agent context:
- Executed-By: agent identity (e.g., beads/crew/dave)
- Rig: the rig name
- Role: crew, polecat, witness, etc.
- Molecule: pinned molecule ID (if any)

Detection sources:
1. GT_ROLE environment variable (set by Gas Town sessions)
2. cwd path patterns (crew/, polecats/, witness/, refinery/)

Trailers are skipped for:
- Human commits (no agent context detected)
- Merge commits (have their own format)
- Commits that already have Executed-By trailer (avoid duplicates)

Executed-By: beads/crew/dave
Rig: beads
Role: crew

cmd/bd/templates/hooks/prepare-commit-msg

@@ -0,0 +1,24 @@
+#!/bin/sh
+# bd-shim v1
+# bd-hooks-version: 0.41.0
+#
+# bd (beads) prepare-commit-msg hook - thin shim
+#
+# This shim delegates to 'bd hooks run prepare-commit-msg' which contains
+# the actual hook logic. This pattern ensures hook behavior is always
+# in sync with the installed bd version - no manual updates needed.
+#
+# Arguments:
+#   $1 = path to the commit message file
+#   $2 = source of commit message (message, template, merge, squash, commit)
+#   $3 = commit SHA-1 (if -c, -C, or --amend)
+
+# Check if bd is available
+if ! command -v bd >/dev/null 2>&1; then
+    echo "Warning: bd command not found in PATH, skipping prepare-commit-msg hook" >&2
+    echo "  Install bd: brew install steveyegge/tap/bd" >&2
+    echo "  Or add bd to your PATH" >&2
+    exit 0
+fi
+
+exec bd hooks run prepare-commit-msg "$@"