fix: Add a changelog policy for TeachLink Web

[Wittig18]

Summary

Added the missing changelog governance document and a regression test, closing the governance gap described in issue #1538.

Files added (2, both inside Governance/):

1. Governance/policies/CHANGELOG_POLICY.md — a new policy document following the house style of the other Governance/policies/*.md files (title, Purpose, Scope, body sections, Ownership, Success). It documents:

   • Changelog format: single root CHANGELOG.md, one ## [MAJOR.MINOR.PATCH] - YYYY-MM-DD section per release, newest-first ordering, an ## [Unreleased] section on top, one single-line past-tense entry per change, linked issue/PR references, **Breaking:** callouts with migration guidance, and security entries that respect the disclosure process.
   • Change categories: the closed list of Added / Changed / Deprecated / Removed / Fixed / Security / Documentation, with tie-breaking rules and a requirement to route new categories through the RFC process.
   • When entries are required: always required for observable changes, written in the same pull request; deferred to release time for default-off feature flags; not required for internal refactors, test-only changes, and no-op formatting; never omitted for convenience.
   • Cross-references to VERSIONING.md, DEPRECATION.md, RELEASE_CHECKLIST.md, VULN_DISCLOSURE.md, and RFC_PROCESS.md.

2. Governance/policies/CHANGELOG_POLICY.test.ts — vitest regression tests (matching the existing DEPRECATION.test.ts / RELEASE_CHECKLIST.test.ts convention) that read the checked-in document and assert its canonical section list, the three required areas, absence of placeholders, the 82-column line-width convention, the format guarantees, the full category list, the entry-requirement rules, and that every referenced governance document actually exists on disk.

No existing files were modified, no functionality was touched, and no changes were made outside Governance/.

Changed files

• Governance/policies/CHANGELOG_POLICY.md
• Governance/policies/CHANGELOG_POLICY.test.ts

Test plan

CI runs the repository's vitest suite. The new Governance/policies/CHANGELOG_POLICY.test.ts will be discovered by the existing vitest config (same pattern as the sibling Governance/policies/DEPRECATION.test.ts and Governance/templates/RELEASE_CHECKLIST.test.ts). Expected: all new tests pass — document structure, format guarantees, category list, entry-requirement rules, and cross-reference existence checks. No existing tests are affected since no source files were changed. Also expect type-check, lint, and build to remain green, as the only additions are a Markdown document and a self-contained test file.

This PR was created as a draft by the issue solver bot. It will remain a
draft until repository CI passes.

Closes #1538

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