docs: note that Codex runs the opt-in session hook

[thisguymartin]

Refs #18. Follow-up to #19.

What changed

#19 said Codex has no plugin hook runtime. That is wrong for Codex 0.157.1. Codex runs hooks/hooks.json from the installed pstack plugin and records trust for it under hooks.state."pstack@open-pstack:hooks/hooks.json:session_start:0:0" in ~/.codex/config.toml. Twenty Codex sessions from 2026-09-27 onward contain the old <EXTREMELY_IMPORTANT> poteto-mode mandate as a developer message, so Codex was auto-firing pstack too.

The opt-in gate from #19 therefore applies to Codex with no extra AGENTS.md block. This PR corrects the README table row, the docs/reference.md tree comment and Codex bullet, and the unreleased CHANGES entry. The bullet also tells Codex users to run codex plugin marketplace upgrade open-pstack after an update. The 0.9.5 changelog entry stays as written because it was accurate then.

No plugin code or skill content changes.

Verification

• Bun tests, strict typecheck, static invariants, and plugin validation pass.
• The exact candidate is installed in every affected harness.
• The changed behavior passes from each real user surface.
• The installed version, action, and observed result appear below.

Local gate: bun run test 198 pass, 0 fail. bun run typecheck clean. PSTACK_STATIC_ONLY=1 bash tests/skill-collision-repro.sh all ok. claude plugin validate plugins/pstack passed.

Codex install: codex plugin marketplace upgrade open-pstack (codex-cli 0.157.1) refreshed the snapshot to 7a8d534. The installed ~/.codex/plugins/cache/open-pstack/pstack/1.4.1/hooks/ matches main byte for byte, and ~/.codex/config.toml is unchanged, including hook trust state.

Live evidence:

Pending. A fresh Codex session should show the <pstack-opt-in> developer message, no pstack skill read for an unrelated small fix, and a poteto-mode read when the request names pstack.

A pull request without live evidence remains a draft. Do not merge, tag, release, or roll it out.