Add Roadmap and WATCH/UNWATCH design docs

[bigmountainben]

Summary

Adds two new documents under docs/:

• Roadmap.md — prioritized list of missing or incomplete RedisRaft features, ranked by likely user impact.
• Watch.md — design for implementing WATCH / UNWATCH on top of the existing session scaffolding.

Both are linked from docs/TOC.md.

Test plan

• Render docs/Roadmap.md and docs/Watch.md on GitHub to confirm formatting.
• Confirm docs/TOC.md links resolve.

🤖 Generated with Claude Code

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