feat(core): add session snapshot and revert system

[thdxr]

Summary

• add a backend-neutral Git service with repository, index, tree, diff, preview, and restore operations
• add per-worktree core snapshot storage and capture settled V2 session step snapshots
• add stateless revert preview and durable revert commit APIs with optional file restoration
• flatten Location service composition and regenerate the V2 JavaScript SDK

Testing

• bun turbo typecheck
• bun test test/snapshot.test.ts test/git.test.ts test/session-projector.test.ts test/session-runner-tool-events.test.ts in packages/core
• bun test test/server/httpapi-public-openapi.test.ts test/server/httpapi-exercise.test.ts in packages/opencode

[greptile-apps]

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