Release Agent: [Unreleased] section not recreated after release

[ashleyshaw]

Issue

After rolling [Unreleased] → [X.Y.Z] - YYYY-MM-DD in CHANGELOG.md during release, the new Unreleased section for the next cycle is never added back.

Impact

• Next PR to develop finds no [Unreleased] section
• Changelog validation fails: "No unreleased changes found"
• Contributors can't add entries to changelog until manually fixed
• Breaks next release cycle

Root Cause

release.agent.js line 499–503 updates CHANGELOG but doesn't inject new Unreleased section afterward.

Expected Behavior

After release, CHANGELOG.md should contain:

## [Unreleased] - DD-MM-YYYY

### Added

### Changed

### Fixed

### Deprecated

### Removed

### Security

### Documentation

### Performance

## [0.5.0] - 2026-05-31
[existing content]

References

• Related audit: .github/reports/audits/RELEASE_PROCESS_AUDIT_v0_5_0.md (Issue 1.3)
• Implementation: scripts/agents/release.agent.js (line 487–514)

[linear-code]

LS-779