Fix agent gate local opcore resolution

[mkceichelbeck]

Summary

Fix the Opcore agent write gate so pre-write validation resolves a repository-local opcore binary when the hook process has a plain session PATH.

Root Cause

The gate spawned opcore validate pre-write by command name only. In installed hook environments, opcore may exist at node_modules/.bin/opcore without that directory being present in PATH. When spawn failed with ENOENT, Node returned nullish stdout and the gate crashed while trimming it.

Changes

• Prepend <repoRoot>/node_modules/.bin to the validation child process PATH while preserving the existing environment and platform delimiter.
• Require the gate-created pre-write request to have a repoRoot before spawning validation.
• Treat nullish validation stdout as empty output so missing command failures report cleanly instead of throwing a TypeError.
• Add regression coverage for repo-local bin resolution and missing-bin no-crash behavior.

Independent Review

Independent reviewer approved the diff with no blocking findings. They specifically checked placement, generated-hook behavior, PATH/Path casing, platform delimiter usage, failure semantics, and test adequacy.

Validation

• git diff --check
• node --test tests/opcore-agent-gate.test.mjs
• npx tsc -b --pretty false
• npm run lint
• PATH="$PWD/node_modules/.bin:$PATH" opcore check changed --base origin/dev --json

Note: full npm run build is blocked in this environment because x86_64-linux-musl-gcc is not installed for the native artifact step.