feat(app): add Open in OMP launch target

[PhilipJohnBasile]

Adds OMP as a first-class launch target alongside the existing coding-agent clients.

• installs or updates only providers.mtplx in the OMP model config
• preserves unrelated providers and backs up recoverable invalid configs
• launches an exact Terminal handoff with token, receipt, cancellation, and cleanup ownership
• reports OMP lifecycle state in the app and identifies OMP requests at the server boundary

Checks:

• swift test --package-path apps/MTPLXApp (666 executed, 1 skipped, 0 failures)
• hosted no-MLX pytest subset (842 passed locally)
• tests/test_server_openai.py (347 passed locally)
• git diff --check