fix: Migrate action-agent.sh to SDK + shared libraries (#5)

Rewrite action-agent from tmux session + phase-handler pattern to
synchronous SDK pattern (agent_run via claude -p). Uses shared libraries:
- agent-sdk.sh for one-shot Claude invocation
- issue-lifecycle.sh for issue_check_deps/issue_close/issue_block
- pr-lifecycle.sh for pr_create/pr_walk_to_merge
- worktree.sh for worktree_create/worktree_cleanup

Add default callback stubs to phase-handler.sh (cleanup_worktree,
cleanup_labels) so it is self-contained now that action-agent.sh
no longer sources it. Update agent-smoke.sh accordingly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dev/phase-handler.sh

@@ -34,6 +34,17 @@ source "$(dirname "${BASH_SOURCE[0]}")/../lib/ci-helpers.sh"
 # shellcheck source=../lib/mirrors.sh
 source "$(dirname "${BASH_SOURCE[0]}")/../lib/mirrors.sh"
 
+# --- Default callback stubs (agents can override after sourcing) ---
+# cleanup_worktree and cleanup_labels are called during phase transitions.
+# Provide no-op defaults so phase-handler.sh is self-contained; sourcing
+# agents override these with real implementations.
+if ! declare -f cleanup_worktree >/dev/null 2>&1; then
+  cleanup_worktree() { :; }
+fi
+if ! declare -f cleanup_labels >/dev/null 2>&1; then
+  cleanup_labels() { :; }
+fi
+
 # --- Default globals (agents can override after sourcing) ---
 : "${CI_POLL_TIMEOUT:=1800}"
 : "${REVIEW_POLL_TIMEOUT:=10800}"