feat(omp): package native prompt-context hook

[andrebrait]

Summary

Package a native OMP before_agent_start extension around the existing prompt-hook CLI. It delivers the CLI's raw tagged context as a hidden native message, including user runs opened by queued messages.

• Uses the installed platform runtime, with an installed PATH executable fallback; never enables the launcher's download fallback.
• Bounds serialized input/output and execution time, rejects unsuccessful/partial output, and cancels obsolete work after session navigation or a newer run.
• Adds OMP entry metadata to both the source package and the actual generated npm thin-installer package; includes installation/removal documentation.
• No query, index, extraction or installer-target changes.

Related work

#442 adds a Pi installer target and CLI guidance. This patch is the separate native OMP startup-hook/package path, not another Pi instruction-file target.

Verification

• Focused native hook/frontload/installer tests:304 pass,3 existing skips.
• Build passes.
• Full suite:4440 pass,12 fail,192 skip. The exact same12 failures reproduced on untouched upstream3ed73bc in7 UI/effect/caller tests; none of those source/test files are changed here. Full suite was run on Node26.8.1, outside the declared <25 range; the comparison used the same environment.
• Actual OMP loader/AgentSession delivers real CodeGraph context from an existing populated index on ordinary prompts, fresh steering and promoted follow-ups (scripted model response only).
• Exercised scripts/pack-npm.sh using the installed runtime dependencies and fresh built dist; generated npm package ships omp/index.ts. The real OMP loader also passed the three run modes against that generated package and its platform runtime.

No existing index was created or rebuilt. Tested host: OMP18.1.17 integration SHA6aef0e8ad51b3bc5ea7a5f2a255c3d48e4c5af72; the version string alone does not guarantee queued-startup fixes. Current isProjectTrusted() is a compatibility API, not an enforced sandbox.

AI-assisted implementation with one independent condensed contract/correctness/test-honesty/simplicity review.