Rewrite CHANGELOG.md with migration guides
- Session Tree: brief description, reference session.md
- Hooks Migration: type renames, event changes, API changes
- Custom Tools Migration: type renames, execute signature, context object
- SDK Migration: type changes, branching API, exports
- RPC Migration: entryId, AgentMessage, compaction events
- Structured Compaction: output format, file tracking
- Interactive Mode: /tree, labels, themes, settings
- Keep Fixed section with external contributions
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