docs: update changelog for v2.5.1

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -6,6 +6,19 @@ Format based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ## [Unreleased]
 
+## [2.5.1] - 2026-03-12
+
+### Added
+- `secure_env_collect` now auto-detects existing keys, destination files, and provides guidance field for better onboarding UX
+
+### Changed
+- Right-sized pipeline for simple work — single-slice milestones skip redundant research/plan sessions, reducing 9-10 sessions to 5-6
+- Heavyweight plan sections (Proof Level, Integration Closure, Observability) are now conditional, omitted for simple slices
+
+### Fixed
+- Squash-merge now aborts cleanly on conflict and stops auto-mode instead of looping with corrupted state
+- Resolved baked-in merge conflict markers in loader.ts, logo.ts, and postinstall.js
+
 ## [2.5.0] - 2026-03-12
 
 ### Added
@@ -230,7 +243,8 @@ Format based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 ### Changed
 - License updated to MIT
 
-[Unreleased]: https://github.com/gsd-build/gsd-2/compare/v2.5.0...HEAD
+[Unreleased]: https://github.com/gsd-build/gsd-2/compare/v2.5.1...HEAD
+[2.5.1]: https://github.com/gsd-build/gsd-2/compare/v2.5.0...v2.5.1
 [2.5.0]: https://github.com/gsd-build/gsd-2/compare/v2.4.0...v2.5.0
 [2.4.0]: https://github.com/gsd-build/gsd-2/compare/v2.3.11...v2.4.0
 [2.3.11]: https://github.com/gsd-build/gsd-2/compare/v2.3.10...v2.3.11