chore: update CHANGELOG for milestone #2

[s2x]

Summary

Update CHANGELOG.md with all changes from milestone #2:

Added

• ServerAction enum for type-safe command actions (feat: introduce ServerAction enum for type-safe command actions #135)
• Pre-push git hook to run composer lint (Add pre-push git hook to run composer lint #137)

Fixed

• TriggerFactory fragile cron expression detection (TriggerFactory — Fragile Cron Expression Detection Heuristic #34, fix(#34): TriggerFactory — Fragile Cron Expression Detection Heuristic #138)
• SupervisorWorker removed sleep(1) hack (SupervisorWorker — sleep(1); exit; Hack #36, fix: remove sleep(1) hack in SupervisorWorker #143)
• WorkermanCompilerPass replaced anonymous class (CompilerPass — Anonymous Class Makes Debugging Difficult #37, fix: replace anonymous class CompilerPass with proper class (#37) #144)

Already documented

• Config validation in ConfigLoader::warmUp() (ConfigLoader — Incomplete Configuration Not Guaranteed #35) - was already in CHANGELOG
• Branch protection on master (Enable branch protection on master branch #132) - was already in CHANGELOG