[Refactor] Remove git from artifact persistence

[samzong]

What's changed?

• Delete artifact/git.ts (commitArtifact, commitArtifactBatch, promise queue)
• Remove simple-git dependency
• Remove gitSha field from Artifact type, Drizzle schema, CREATE TABLE SQL, search, save, stores
• Remove git init + .gitignore creation from workspace setup
• Remove git commit calls from artifact handlers and auto-extract
• Remove empty Git tab from RightPanel
• Update all architecture docs, project guides, and website copy to reflect two-layer persistence (filesystem + SQLite)

Why

• Artifact filenames include UUIDs — files are never modified after creation, making git diffs and version history meaningless
• gitSha was stored in DB but never consumed by any UI or query
• SQLite + filesystem already handle persistence, search, and traceability
• Removes simple-git dependency, I/O overhead, and serialization complexity

[github-actions]

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