docs: spec-kit pilot for state-sync component

[jeremyeder]

Summary

• Adds behavioral specification for the state-sync sidecar (components/runners/state-sync/spec/spec.md)
• Adds development context file (.claude/context/state-sync-development.md) following existing pattern
• Adds BOOKMARKS.md entries for progressive disclosure

Pilots spec-driven development on the smallest runner component before rolling out to ambient-runner.

Test plan

• Verify BOOKMARKS.md links resolve to real files
• Review spec invariants against hydrate.sh and sync.sh source
• Review development context format against backend-development.md

🤖 Generated with Claude Code

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[markturansky]

@jeremyeder this is an excellent extraction of the runner spec. It has all the config/params for the runner and backend APIs required by it.

I think it would be useful to also specify the runtime of the runner itself. It has multiple threads internally, acting as source and sink. There are bridges to other models besides Claude. We'll add gRPC to the spec as well (when we merge back from alpha).