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>

.woodpecker/agent-smoke.sh

@@ -199,9 +199,9 @@ check_script lib/ci-debug.sh
 check_script lib/parse-deps.sh
 
 # Agent scripts — list cross-sourced files where function scope flows across files.
-# phase-handler.sh calls helpers defined by its sourcing agent (action-agent.sh).
+# phase-handler.sh defines default callback stubs; sourcing agents may override.
 check_script dev/dev-agent.sh
-check_script dev/phase-handler.sh      action/action-agent.sh lib/secret-scan.sh
+check_script dev/phase-handler.sh      lib/secret-scan.sh
 check_script dev/dev-poll.sh
 check_script dev/phase-test.sh
 check_script gardener/gardener-run.sh
@@ -215,7 +215,7 @@ check_script vault/vault-fire.sh
 check_script vault/vault-poll.sh
 check_script vault/vault-reject.sh
 check_script action/action-poll.sh
-check_script action/action-agent.sh    dev/phase-handler.sh
+check_script action/action-agent.sh
 check_script supervisor/supervisor-run.sh
 check_script supervisor/preflight.sh
 check_script predictor/predictor-run.sh