fix(runtime): verify required artifact readability

[BunsDev]

Summary

• keep injected command-stat probes scoped to executable discovery
• verify required launch artifacts with the real readability check unless a dedicated readable-file test double is supplied
• add deterministic regression coverage for the post-merge Copilot finding on fix(opencode): preflight a shell-free launch plan #3927

Verification

• red regression confirmed against the old statFile fallback
• focused runtime-availability.test.ts passes on current origin/main
• pnpm typecheck
• pnpm lint
• pnpm check:tests-wired — 1,274 files wired, 1 allowlisted
• git diff --check origin/main...HEAD
• pnpm test:api — 279/279 files passed before the overlapping Hermes fix(chat): preflight Hermes direct launch safely #3911 base advance; on exact current base, the suite reaches an existing route-hermes-availability.integration.test.ts failure (runtime_process_failed vs runtime_missing) that reproduces identically on a clean detached origin/main checkout at 25d817c0b

Follow-up to #3927.

[Copilot]

Pull request overview

Tightens evaluateRuntimeAvailability() preflight semantics so required launch artifacts are verified using a real readability check by default (rather than allowing a command statFile test double to silently satisfy artifact checks), and adds regression tests for the behavior discovered post-merge in #3927.

Changes:

• Remove the implicit fallback that reused statFile as the required-artifact readability check; required artifacts now default to defaultReadableFile.
• Extend Windows shim/direct-launch coverage to ensure a statFile double can’t mask missing/unreadable required artifacts.
• Add OpenCode Windows required-artifact readiness coverage using an explicit readableFile double.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/lib/runtime-availability.ts	Ensures required-artifact verification uses defaultReadableFile unless an explicit readableFile probe is provided.
src/lib/runtime-availability.test.ts	Adds regression tests covering required-artifact readability (including OpenCode Windows scenarios).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.