refactor: extract run admission runtime

[ByBrawe]

Summary

Continue the V1 executor modularization by moving due selection and pre-dispatch admission policy out of runtime/executor.js.

• add runtime/run-admission.js for due-job selection, max-runtime stop, stop-file stop, --until, and preflight safe/failure handling;
• keep executor focused on active-run ownership, session/compaction coordination, branch/compaction setup, dispatch state, and recovery;
• preserve the existing executor injection surface by forwarding workspace/host/process dependencies into the admission runtime;
• add direct cross-platform regressions for due selection, stop conditions, dangerous preflight rejection, failing preflight, and successful admission.

Behavior-preserving source modularization only; no runtime state schema or user-visible command behavior change. The committed generated src/index.js remains behavior-equivalent and unchanged in this source-only slice; the Bundle Gate regenerated the exact source head and exercised that generated bundle against real OpenCode on Linux and Windows.

Verification

Exact head d1203ea998267731014ab4e9c0340d9911d42729:

• CI: success after one unchanged retry of the Ubuntu test job. Its first attempt hit a timing-sensitive comprehensive-harness assertion; Windows, both compatibility lanes, both real-host lanes, and the generated-bundle regression suite were already green. The Ubuntu retry passed unchanged.
• Single-file Bundle Gate: success on Linux and Windows, including source regeneration, generated entry import, real-host Loop/queued-Goal canaries, and generated-bundle regression suite.
• V2 Lifecycle Contract: success.

The first branch head also exposed a Windows-only path expectation in the new stop-file unit fixture; the fixture was corrected to compare against the platform-resolved path. No production runtime change was needed for that failure.

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