fix: improve session diff viewing infrastructure

[dylsteck]

Summary

This PR improves the existing session right-sidebar Git tab diff infrastructure. It does not add a new review page, modal, or separate review surface.

• Upgrades @pierre/diffs to ^1.2.5.
• Replaces the per-file PatchDiff path with virtualized CodeView rendering.
• Parses raw patches with parsePatchFiles instead of a regex splitter.
• Enables Pierre's worker-pool rendering path for deferred syntax highlighting.
• Versions parsed patch cache keys and controlled CodeView items from patch contents to avoid stale worker-rendered output.
• Preserves the existing dark unified layout, wrapped lines, file status labels, additions/deletions, empty states, no-text-patch fallbacks, and truncated patch warning.

Backend

• Keeps GET /chat/:sessionId/git/state response shape stable: diff.patch, diff.truncated, diff.files, diff.additions, diff.deletions.
• Keeps local sandbox diffs on the existing git diff ... | head -c 250000 path.
• Moves GitHub PR file/commit/diff collection into a focused helper module.
• Fetches GitHub PR diff text through an Effect program with typed GitHubRepoError / StreamWriteError failures.
• Bounds PR diff reads to MAX_PATCH_BYTES + 1, cancels the stream on truncation, and falls back to bounded response.text() when GitHub returns no stream body.
• Continues using Octokit pulls.listFiles for file metadata and pulls.listCommits for commit rows.
• Avoids logging patch contents or tokens.

Tests / Verification

• Multi-agent review completed before staging; findings were addressed before commit.
• pnpm --filter web test -- git-tab-diff-model.test.ts
• pnpm --filter api test -- github-pr-diff.test.ts session-git-state.test.ts
• pnpm --filter web typecheck
• pnpm --filter api typecheck
• pnpm build && pnpm lint && pnpm typecheck && pnpm test
• pnpm openapi:check
• pnpm sdk:generate:check

Reference with many learnings that were used to help shape this PR: https://pierre.computer/writing/on-rendering-diffs