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>
