Harden launcher preflight and port handling

[namabeeru]

Summary

• Reintroduce the runtime-preflight and launcher-hardening parts from Defer provider SDK imports and harden startup launchers #347 after the model-factory import fix landed separately in Defer OpenAI/Anthropic SDK imports in model factory #348.
• Make dependency probing fail open when the probe is inconclusive, while still exiting early when missing runtime dependencies are genuinely confirmed.
• Derive launcher and shortcut URLs from --frontend-port / --backend-port instead of hardcoding the backend port.
• Centralize the startup ready marker so craftbot.py, run.py, and installer/api.py share the same value.

Response to review feedback

@korivi-CraftOS this addresses the two follow-up items from #347:

• runtime_preflight.py: probe infrastructure failures now warn and continue. TimeoutExpired, subprocess exceptions, nonzero probe exits, missing sentinel output, malformed JSON, and non-list JSON are all treated as inconclusive. Startup only exits when the probe succeeds and returns a concrete list of missing imports.
• 60s timeout rationale: I kept it as a conservative upper bound for slower conda/mamba startup plus import-heavy dependency checks. The important change is that a timeout is now non-fatal and only prevents the probe from hanging startup indefinitely.
• craftbot.py: frontend/backend URLs now derive from the passed port flags, including both --backend-port 8123 and --backend-port=8123 forms. The macOS shortcut uses the derived backend URL for its health check.
• Ready marker duplication: CRAFTBOT IS READY now lives in top-level startup_constants.py, which both the installer and agent-side code can import.

Validation

• /usr/local/bin/python3.10 -m pytest tests/test_run_dependency_check.py tests/test_craftbot_service.py tests/test_startup_constants_usage.py tests/test_model_factory.py -q -> 15 passed
• Focused PR suite looped 10/10 successfully before opening this PR.
• /usr/local/bin/python3.10 -m py_compile craftbot.py run.py app/main.py app/runtime_preflight.py installer/api.py startup_constants.py tests/test_craftbot_service.py tests/test_run_dependency_check.py tests/test_startup_constants_usage.py -> passed
• git diff --check origin/main...HEAD -> passed
• Bad-runtime loop with Homebrew Python 3.14.5: run.py, python -m app.main, and craftbot.py start --cli all exited nonzero with clear missing-dependency messaging; no traceback, false CRAFTBOT STARTED, or stale PID.
• Good-runtime preflight loop with /usr/local/bin/python3.10: 10/10 successful.
• Real launcher smoke with /usr/local/bin/python3.10: 3/3 start/status/stop cycles successful.

Repo-wide checks still have unrelated existing failures outside this PR's changed files:

• /usr/local/bin/python3.10 -m pytest tests -q -> 101 passed, 4 failed, 16 deselected; the failures are in tests/test_trigger_router_and_parking.py because FakeLLM.generate_response_async() does not accept prompt_name.
• /usr/local/bin/python3.10 -m pytest -q -> collection errors in generated/template Living UI test directories (models import and tests.conftest import-path mismatch).

[namabeeru]

Retargeted this PR to dev to match the contributing guide (dev -> staging -> main).