Wire mix stokowski up to actually launch a session

[bougyman]

Goal

Close out the follow-up from #100/#101: make mix stokowski actually launch a session instead of only validating workflow.yaml.

What changed

• mix stokowski now runs uv run --project vendor/stokowski --extra web --with-editable vendor/stokowski -- stokowski <workflow> <args...> after its existing safety checks pass. Flags are passed straight through (--dry-run, --port, --host, -v).
• --with-editable is required alongside --project: vendor/stokowski's pyproject.toml has no [build-system] table, so a plain uv run --project only syncs dependencies and never installs the stokowski package/entry point itself - confirmed directly (uv run --project vendor/stokowski -- stokowski --help fails with Failed to spawn: stokowski even though the venv builds cleanly and installs 23 dependency packages).
• Verified end-to-end against the real workflow.yaml and a real LINEAR_API_KEY: config validated, correctly resolved the env-var-fallback API key, hit the real Linear API (200 OK), correctly used the neutral workspace.root fixed in feat: vendor stokowski for Linear-driven agent orchestration #101.

Follow-up (still not in this repo)

The missing [build-system] table is an upstream packaging gap in Sugar-Coffee/stokowski itself - this PR works around it locally via --with-editable, but it should be fixed upstream too. Rolling this into the same eventual upstream PR as the LINEAR_API_KEY resolution fix (from #100/#101's plan) rather than opening a separate one.