docs: require confirmation for release notes changes

[JJasonSun]

Summary

- Update changelog as part of API documentation sync.
+ Present new entries and substantive edits for explicit user or maintainer confirmation.

Routine documentation work does not automatically add release notes. Other documentation work continues while confirmation is pending.

API contract

Not changed.

Evidence

• Before: AGENTS.md included changelog in the API documentation update list.
  After: It requires confirmation of proposed additions and substantive edits before writing them.
• git diff --check passed; the diff contains only AGENTS.md.

Checklist

• I kept the change focused and public-facing.
• No secrets, private URLs, phone numbers, or customer data were added.
• Any OpenAPI change describes supported public behavior — not applicable; OpenAPI is unchanged.
• I updated tests when observable site behavior changed — not applicable.
• I added screenshots for visual changes — not applicable.
• I ran pnpm run validate — not run; this change only updates agent instructions.

Merge Danger

Door: Two-way. Reverting restores the previous instructions.

Blast Radius: Documentation. Changes the agent's release-notes workflow.