docs: link Swarm AI's optional COMA API use

[lukisch]

Summary

• Add reciprocal Swarm AI references to COMA's English, German, and machine-readable ecosystem documentation, describing Swarm's optional use of COMA's Spawner and adapter APIs.
• Add portable tests for provider-free session dry-runs and Codex/Kimi launcher command construction, including explicit Windows Node and native executable contracts plus host-default behavior.
• No runtime dependency or production-code changes.

CI context

The earlier CI failures came from test assumptions: the dry-run test required an installed codex CLI, and Windows-specific Node entrypoint assertions ran unconditionally on POSIX. The tests now use a controlled fake executable, prohibit subprocess launches in dry-run cases, assert the missing-CLI exit 2 contract, and cover explicit and host-default launcher behavior.

Validation

• Focused session and adapter tests: 43 passed.
• Full test suite: 308 passed.
• Ruff 0.16.8: ruff check . passed.
• git diff --check and staged-diff privacy scan passed.

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