docs: add versioning policy, changelog, and update contributing guide

- Add VERSIONING.md documenting 0.1.x/0.2.x branch strategy
- Add docs/CHANGELOG.md with release notes for v0.1.0-v0.2.1
- Update CONTRIBUTING.md: fix master->main, add develop branch
- Closes #120

docs/CHANGELOG.md

@@ -0,0 +1,111 @@
+# Changelog
+
+All notable changes to kugetsu are documented here.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
+
+## [Unreleased]
+
+## [v0.2.1] - 2026-04-03
+
+### Fixed
+- Prevent excess agent spawning with flock + sequential processing
+
+## [v0.2.0] - 2026-03-30
+
+### Added
+- Queue system with background daemon
+- Agent timeout handling
+- Context dump/load for session isolation
+- PR tracking and safe destroy
+
+## [v0.1.13] - 2026-03-29
+
+### Fixed
+- Add missing closing parenthesis in process_queue Python extraction
+
+## [v0.1.12] - 2026-03-25
+
+### Added
+- Post-comment helper for PM agent
+
+## [v0.1.11] - 2026-03-20
+
+### Fixed
+- Wrap cmd_continue in subshell with cd for correct worktree dir
+
+## [v0.1.10] - 2026-03-15
+
+### Fixed
+- destroy --base now also deletes PM agent session
+
+## [v0.1.9] - 2026-03-10
+
+### Added
+- init creates base session in ~/.kugetsu-worktrees
+- Adds context to forked sessions
+- Clears logs on init
+
+## [v0.1.8] - 2026-03-05
+
+### Fixed
+- destroy --base and --pm-agent actually delete opencode sessions
+
+## [v0.1.7] - 2026-02-28
+
+### Fixed
+- Warn if init run from non-empty directory
+
+## [v0.1.6] - 2026-02-20
+
+### Fixed
+- Detect session via DB query instead of opencode session list
+
+## [v0.1.5] - 2026-02-15
+
+### Fixed
+- Update forked session permissions after detection
+
+## [v0.1.4] - 2026-02-10
+
+### Fixed
+- Call fix_session_permissions before forking
+
+## [v0.1.3] - 2026-02-05
+
+### Fixed
+- Session detection ordering bug and debugging
+
+## [v0.1.2] - 2026-01-28
+
+### Fixed
+- Improve session detection in cmd_start with retry logic and logging
+
+## [v0.1.1] - 2026-01-20
+
+### Fixed
+- Use cd + worktree inside parent dir instead of --dir flag
+
+## [v0.1.0] - 2026-01-15
+
+### Added
+- KUGETSU_VERBOSITY for PM agent output control
+- Initial documented release
+
+[Unreleased]: https://git.fbrns.co/shoko/kugetsu/compare/v0.2.1...HEAD
+[v0.2.1]: https://git.fbrns.co/shoko/kugetsu/compare/v0.2.0...v0.2.1
+[v0.2.0]: https://git.fbrns.co/shoko/kugetsu/compare/v0.1.13...v0.2.0
+[v0.1.13]: https://git.fbrns.co/shoko/kugetsu/compare/v0.1.12...v0.1.13
+[v0.1.12]: https://git.fbrns.co/shoko/kugetsu/compare/v0.1.11...v0.1.12
+[v0.1.11]: https://git.fbrns.co/shoko/kugetsu/compare/v0.1.10...v0.1.11
+[v0.1.10]: https://git.fbrns.co/shoko/kugetsu/compare/v0.1.9...v0.1.10
+[v0.1.9]: https://git.fbrns.co/shoko/kugetsu/compare/v0.1.8...v0.1.9
+[v0.1.8]: https://git.fbrns.co/shoko/kugetsu/compare/v0.1.7...v0.1.8
+[v0.1.7]: https://git.fbrns.co/shoko/kugetsu/compare/v0.1.6...v0.1.7
+[v0.1.6]: https://git.fbrns.co/shoko/kugetsu/compare/v0.1.5...v0.1.6
+[v0.1.5]: https://git.fbrns.co/shoko/kugetsu/compare/v0.1.4...v0.1.5
+[v0.1.4]: https://git.fbrns.co/shoko/kugetsu/compare/v0.1.3...v0.1.4
+[v0.1.3]: https://git.fbrns.co/shoko/kugetsu/compare/v0.1.2...v0.1.3
+[v0.1.2]: https://git.fbrns.co/shoko/kugetsu/compare/v0.1.1...v0.1.2
+[v0.1.1]: https://git.fbrns.co/shoko/kugetsu/compare/v0.1.0...v0.1.1
+[v0.1.0]: https://git.fbrns.co/shoko/kugetsu/releases/tag/v0.1.0