docs: document Changelog Panel safety boundaries and contributor handoff

[emarc99]

• Closes [Existing app][Changelog Panel] Document safety boundaries and contributor handoff #1000

Summary

Resolves issue by establishing comprehensive safety, privacy, and contributor handoff documentation for the existing Changelog Panel feature module (src/features/changelog/).

Changes Implemented

1. Feature Module Handoff Documentation (src/features/changelog/README.md):

   • Files & Architecture Map: Details purpose and roles of ChangelogPanel.tsx, useChangelog.ts, data.ts, helpers.ts, types.ts, and index.ts.
   • Data Contracts: Explicit definitions for ChangelogEntry interface, category types (ui, api, protocol, security), and grouping format.
   • User-Facing States: Documents first-visit state, green unread dot indicators, unread card highlighting, "All read" status badge, empty release state, and external link accessibility.
   • Safety & Privacy Boundaries: Documents demo data hygiene policy, zero real user data/secrets rules, localStorage (stealth:changelog:seen-version) fallback handling in private browsing mode, and client-side privacy.
   • Contributor QA Checklist: Step-by-step UI, keyboard navigation, screen reader, theme contrast, and test checklist.
   • Local References: Clickable markdown links to local unit test suite tests/unit/changelog/changelog-helpers.test.ts and E2E test suite tests/e2e/changelog.spec.ts.

2. Code & Data Annotations (src/features/changelog/data.ts):

   • Added JSDoc safety comments above CHANGELOG_ENTRIES documenting static demo data policies and data hygiene rules.

Fixes #1000