feat(hooks): implement thin shim pattern (GH#615)

Replace full shell script hooks with thin shims that delegate to
'bd hooks run <hook-name>'. This eliminates hook version drift -
upgrading bd automatically updates hook behavior.

Changes:
- Add 'bd hooks run' subcommand with hook logic in Go
- Convert templates to minimal shims (~17 lines each)
- Shims use '# bd-shim v1' marker (format version, not bd version)
- Update version checking to recognize shim format
- Shims are never marked as "outdated"

Benefits:
- No more manual 'bd hooks install --force' after upgrades
- Hook logic always matches installed bd version
- Follows pattern used by husky, pre-commit, lefthook

Migration: Run 'bd hooks install --force' one final time.

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

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

cmd/bd/templates/hooks/post-merge

@@ -1,56 +1,18 @@
 #!/bin/sh
-# bd-hooks-version: 0.30.7
+# bd-shim v1
 #
-# bd (beads) post-merge hook
+# bd (beads) post-merge hook - thin shim
 #
-# This hook syncs the bd database after a git pull or merge:
-# 1. Checks if any .beads/*.jsonl file was updated
-# 2. Runs 'bd sync --import-only --no-git-history' to import changes
-#
-# Installation:
-#   cp examples/git-hooks/post-merge .git/hooks/post-merge
-#   chmod +x .git/hooks/post-merge
-#
-# Or use the install script:
-#   examples/git-hooks/install.sh
-
-# Skip during rebase - git may run post-merge during rebase operations
-# and we must not modify working tree files or the rebase will fail
-if [ -d .git/rebase-merge ] || [ -d .git/rebase-apply ]; then
-    exit 0
-fi
+# This shim delegates to 'bd hooks run post-merge' which contains
+# the actual hook logic. This pattern ensures hook behavior is always
+# in sync with the installed bd version - no manual updates needed.
 
 # Check if bd is available
 if ! command -v bd >/dev/null 2>&1; then
-    echo "Warning: bd command not found, skipping post-merge sync" >&2
+    echo "Warning: bd command not found in PATH, skipping post-merge hook" >&2
+    echo "  Install bd: brew install steveyegge/tap/bd" >&2
+    echo "  Or add bd to your PATH" >&2
     exit 0
 fi
 
-# Check if we're in a bd workspace
-if [ ! -d .beads ]; then
-    # Not a bd workspace, nothing to do
-    exit 0
-fi
-
-# Check if any JSONL file exists in .beads/
-if ! ls .beads/*.jsonl >/dev/null 2>&1; then
-    exit 0
-fi
-
-# Run bd sync --import-only --no-git-history to import the updated JSONL
-# --no-git-history prevents writes to deletions.jsonl during hook operations
-# This is more robust than direct import as it handles all edge cases
-# Capture both stdout and stderr to show user what went wrong
-if ! output=$(bd sync --import-only --no-git-history 2>&1); then
-    echo "Warning: Failed to sync bd changes after merge" >&2
-    echo "$output" >&2
-    echo "" >&2
-    echo "Run 'bd doctor --fix' to diagnose and repair" >&2
-    # Don't fail the merge, just warn
-fi
-
-# Run quick health check (silent on success, hints if issues found)
-# This catches version mismatches, outdated hooks, etc.
-bd doctor --check-health 2>/dev/null || true
-
-exit 0
+exec bd hooks run post-merge "$@"