fix(snapshot): let git drop ignored snapshot paths

[thdxr]

Summary

• simplify snapshot staging to rely on git's index queries instead of manual ignore parsing
• restore tracked paths that are now ignored back to the previous snapshot tree using --pathspec-from-file=- and --pathspec-file-nul so large ignored sets do not overflow argv on Windows
• remove redundant ignore filtering from patch() and diffFull() now that add() keeps the snapshot index clean

Testing

• bun test test/snapshot/snapshot.test.ts
• bun typecheck
• bun test (currently fails in test/session/session.test.ts: session.created event should be emitted before session.updated)

[greptile-apps]

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