docs: add 1.2.0 changelog section for release

[adlio]

This pull request was created by @kiro-agent on behalf of @adlio 👻

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Summary

Adds the [1.2.0] release section to CHANGELOG.md. This entry was authored on the PR #10 branch but was not included in the merge (PR #10 merged at the commit prior to the changelog commit), so main currently lacks a [1.2.0] section.

This is required before tagging v1.2.0, because the release workflow (.github/workflows/release.yml) extracts release notes by matching the ## [1.2.0] header in CHANGELOG.md. Without this section, the published release would have empty notes.

Changes

• Add [1.2.0] - 2026-07-09 section documenting:
  • Security: opencontainers/runc v1.2.3 -> v1.2.8 (CVE-2025-31133, CVE-2025-52565, CVE-2025-52881, CVE-2026-41579)
  • Fixed: flaky TestApplyInLexicalOrder timing assertion
  • Known Issues: remaining pgx/v4 and pgproto3/v2 vulnerabilities requiring pgx/v5 migration
• Update changelog comparison links ([Unreleased] -> v1.2.0...HEAD, add [1.2.0] link)

Testing

Documentation-only change. The section header matches the release workflow's extraction pattern.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ee514e4) to head (4c6135b).

Additional details and impacted files

@@            Coverage Diff            @@
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  Coverage   100.00%   100.00%           
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  Files            7         7           
  Lines          216       216           
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  Hits           216       216           

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