docs(skills): require gate for file approval

[zhangjinzan1]

Summary

• distinguish ordinary annotation from explicit file approval in the core skills
• require --gate --json when a user asks to approve a saved plan/spec/document
• clarify that --json alone does not enable the Approve button
• add a regression assertion for the approval-gate guidance

Why

A Codex agent followed the installed core skill and launched a generated plan with plannotator annotate <file> --markdown --json. The UI correctly opened in plain annotate mode, so it showed Close but no Approve, even though the agent told the user they could approve the plan.

Test

• bun test apps/hook/server/plannotator-skill-reference.test.ts (6 passed, 0 failed)
• git diff --check

[backnotprop]

Merged. Good catch on the gate/json distinction, and thanks for wiring the guidance into the freshness test so it cannot drift. This will ride the next release.