Add guided local setup workflow

[aidankhogg]

Motivation

• Provide a single-command, first-time guided setup that sequences host preflight checks, compose startup, Postgres health wait, migrations, spec-aware doctor checks, and then continues into the existing bootstrap flow to simplify and standardize local onboarding.
• Surface actionable remediation for common blockers (subnet/port/Docker name/database) so users know how to fix failures before running netengine up.
• Avoid requiring real infrastructure in CI by adding mocked tests for the orchestration path so the workflow can be validated without Docker/Postgres.

Description

• Add a new CLI command netengine setup local SPEC_FILE and async handler _setup_local which orchestrates: pre-Postgres host checks, docker compose up -d postgres, waiting for Postgres health, running migrations, running spec-aware doctor checks, and then calling the existing _up path with migrations skipped. (netengine/cli/main.py)
• Implement helper probe and orchestration utilities: _setup_host_probes, _run_setup_host_checks, _docker_compose_up, _wait_for_postgres_health, _required_setup_failed, and _print_setup_results, plus a LOCAL_SETUP_DB_URL fallback for common local setups. (netengine/cli/main.py)
• Replace the long manual bootstrap command sequences in README.md, docs/alpha-quickstart.md, and docs/runbook.md with the new guided netengine setup local workflow and add remediation hints for common failures. (README.md, docs/alpha-quickstart.md, docs/runbook.md)
• Add unit/CLI tests that mock Docker/DB/doctor calls and verify orchestration success and failure-stop behavior. (tests/test_cli.py, small test fix in tests/test_doctor.py)

Testing

• Ran the new and related CLI unit tests with pytest tests/test_cli.py tests/test_readiness_cli.py tests/test_doctor.py -q and they passed (all targeted tests succeeded).
• Verified the setup help output with python -m netengine.cli.main setup --help which produced the expected usage.
• Ran the full test suite with pytest -q; the suite completed with the majority of tests passing but two existing integration tests failed (unrelated to the setup workflow): TestFullMVPLifecycle.test_full_mvp_lifecycle and TestHealthRoute.test_health_does_not_leak_last_error_detail, which remain outside the scope of this change.

---

Codex Task