docs: Document ULOOP_PROJECT_RUNNER_PATH project runner override

[hatayama]

Summary

ULOOP_PROJECT_RUNNER_PATH (defined in cli/dispatcher/internal/nativepath/path.go, consumed first in resolveDispatcherRealCLI) lets the dispatcher run a locally built project runner instead of resolving one from the pin. It is essential for dogfooding checkouts — release-please stamps projectRunnerVersion ahead of the published GitHub release, so the normal download path 404s — yet no document in the repository mentioned it.

Changes

• docs/project-runner-pin.md: new "Local override: ULOOP_PROJECT_RUNNER_PATH" section describing precedence (checked before pin validation, sibling binary, cache, and download), the executable-file requirement, the dogfooding rationale, and the sibling ULOOP_INSTALL_DIR / ULOOP_CACHE_DIR overrides.
• AGENTS.md (Native Go CLI Validation): one-paragraph pointer for validating unreleased Go changes from an external Unity project.

Docs-only change; no code, scripts, or shared release inputs touched.

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: cb7105e8-82da-4349-adf3-a011fa919016

📥 Commits

Reviewing files that changed from the base of the PR and between 7ac8325 and eb10e42.

📒 Files selected for processing (2)

• AGENTS.md
• docs/project-runner-pin.md

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📝 Walkthrough

Walkthrough

Documents ULOOP_PROJECT_RUNNER_PATH for running a locally built project runner, including precedence, validation, bypassed pin-resolution mechanisms, and external Unity project validation guidance.

Changes

Local project-runner override

Layer / File(s)	Summary
Document local runner resolution AGENTS.md, docs/project-runner-pin.md	Documents the ULOOP_PROJECT_RUNNER_PATH override, its executable-path validation, bypassed pin and download mechanisms, and the related dogfooding workflow.

Estimated code review effort: 1 (Trivial) | ~3 minutes

Possibly related PRs

• hatayama/unity-cli-loop#1863: Implements the dispatcher override behavior documented here.
• hatayama/unity-cli-loop#1859: Updates the pinned dispatcher release used when no local override is configured.
• hatayama/unity-cli-loop#1674: Documents pin-driven project-runner selection and related override limitations.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main docs-only change about the ULOOP_PROJECT_RUNNER_PATH override.
Description check	✅ Passed	The description matches the change set and accurately describes the new documentation and validation guidance.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/project-runner-path-override

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