Prevent pstack runner from auto-loading project env files

[rajinsyed]

Problem

The shipped pstack-runner uses a plain Bun shebang. When launched from a project directory, Bun automatically loads that project's .env files before runner code executes. A project-level provider key can then override an authenticated CLI subscription.

Expected behavior

The runner must not auto-load project env files. It should continue to honor environment variables explicitly inherited from the parent process.

Verification

Add an executable-boundary regression with a sentinel .env, run the runner/typecheck suites, and install/test the exact candidate from the Codex surface.

[rajinsyed]

Local candidate ddfd5cb changes the runner shebang to #!/usr/bin/env -S bun --no-env-file, documents direct invocation, and adds an executable-boundary regression. The test was red before the change (.env sentinel leaked) and green after; it also proves an explicitly inherited sentinel survives.

Verification on 2026-08-26:

• bun run test: 102 passed, 453 assertions
• strict typecheck: passed
• static invariants/manifests: passed
• exact candidate copied into the active Codex pstack 1.1.0 cache
• from a real repository containing ANTHROPIC_API_KEY in .env, parent key remained unset, claude auth status reported claude.ai / max, and a direct Fable lane returned SUBSCRIPTION_OK with model verification

No environment-key values were read or logged.