docs: define the W-33 identity and sync model

[enu3379]

Summary

Separates the W-33 document identity and sync-family design from the Figure UX feature work. This PR establishes the v2.1 design source of truth before runtime implementation begins.

Closes #19
Refs #1

Changes

• Add docs/doc-identity-sync.md v2.1
• Define document nodes, locators, content evidence, annotation buckets, and sync-family states
• Define file-only download, memo-inclusive download, clone, and export semantics
• Define conflict handling, deletion lifecycle, implementation tasks, and UI/QA ownership
• Link the design source of truth from docs/progress.md

Scope

Documentation only. No runtime behavior, manifest, storage schema, or dependency changes are included. The existing feature/w33-identity-sync worktree and its uncommitted docs/w33-ui-spec.md changes are intentionally untouched.

Test plan

• npm run typecheck
• npm test — 6 files, 25 tests
• npm run build
• git diff --check

Checklist

• PR title follows Conventional Commits
• AI-assisted

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