docs(changelog): retitle Unreleased to 0.13.0 (released 2026-07-30)

[mavaali]

Release commit bf97990 (chore: release v0.13.0, #58) bumped pyproject.toml to 0.13.0 but never retitled the ## [Unreleased] section in CHANGELOG.md, leaving all 0.13.0-era entries under Unreleased.

This retitles the section to ## [0.13.0] - 2026-07-30 (the release date of bf97990) and adds a fresh empty ## [Unreleased] above it, matching the file's Keep a Changelog conventions. CHANGELOG.md is unchanged since bf97990, so every entry in the section belongs to 0.13.0 — no post-release entries needed to stay under Unreleased.

Resolves the changelog finding from the nightly maintenance sweep in #60.

🤖 Generated with Claude Code

[claude]

Reviewed for correctness against the invariants in GUIDE.md / docs/boundary-contract-spec.md (write allowlist/floor/ceiling, staging pivot, spend-fails-closed, Third Umpire grading, selftest coverage, red-team tests, core-vs-overlay boundary) — none of those apply here, this PR only retitles ## [Unreleased] to ## [0.13.0] - 2026-07-30 in CHANGELOG.md.

• Change is a correct, minimal 2-line diff matching Keep a Changelog conventions; pyproject.toml is already at 0.13.0, consistent with the new heading.
• No code, tests, or enforcement logic touched, so no selftest/red-team coverage is needed.
• Pre-existing: the version reference-link list at the bottom of CHANGELOG.md hasn't been maintained past [0.5.0] (no [0.13.0]: .../releases/tag/v0.13.0 link), but that gap predates this PR and is out of scope for a retitle-only change.

No correctness, simplification, or efficiency findings. LGTM.