feat: add git working tree diff toggle in diff panel

[D3OXY]

Closes #1590

Summary

Adds a toggle in the diff panel header that switches between two modes:

• Session mode (default) — existing checkpoint-based turn-by-turn diffs, unchanged behavior.
• Git mode — live git diff HEAD output showing all staged + unstaged changes on tracked files.

This gives visibility into changes made outside the agent (manual edits, IDE refactors, other tools) and provides a reliability fallback when checkpoint-based diffs are incomplete or unavailable.

What changed

Contracts (packages/contracts)

• Added GitWorkingTreeDiffInput / GitWorkingTreeDiffResult schemas in git.ts
• Added git.workingTreeDiff RPC method definition in rpc.ts
• Added workingTreeDiff to the NativeApi.git interface in ipc.ts

Server (apps/server)

• Added readWorkingTreeDiff to the GitCoreShape service interface
• Implemented it in the GitCore layer: runs git diff HEAD --patch --minimal --no-color, with a fallback to git diff --cached for repos with no commits yet. Output capped at 512KB with truncation.
• Wired the git.workingTreeDiff RPC handler in ws.ts

Web (apps/web)

• Added gitWorkingTreeDiffQueryOptions React Query helper in gitReactQuery.ts
• Added workingTreeDiff to the RPC client and NativeApi wiring
• Added diffScope: "session" | "git" URL search param in diffRouteSearch.ts
• Updated DiffPanel.tsx:
  • New session/git toggle group in the header (list icon / git branch icon), placed before the existing stacked/split toggle
  • In session mode: existing turn chip strip (unchanged)
  • In git mode: turn strip hidden, replaced with a "Working tree changes" label
  • Both modes share the same diff renderer, word-wrap, and stacked/split controls
  • Live refresh: the git diff query is automatically invalidated whenever the existing git status subscription detects working tree changes — no polling needed

Tests

• Added 3 test cases for diffScope parsing in diffRouteSearch.test.ts

UI

Design decisions

• Refresh strategy: piggybacks on the existing subscribeGitStatus stream. When the server broadcasts a git status change, the diff query is invalidated and refetches. Zero polling overhead.
• Output cap: 512KB max patch output, truncated with [truncated] marker. Larger than readRangeContext's 59KB cap since working tree diffs tend to be bigger, but still bounded.
• No-commit repos: falls back to git diff --cached to handle initial-add scenarios gracefully.
• Tracked files only: git diff HEAD covers staged + unstaged changes on tracked files. Untracked files are excluded (consistent with checkpoint diffs).
• URL-persisted scope: diffScope is stored in the URL search params so the mode survives navigation and can be deep-linked.

Test plan

• Open diff panel → verify session mode works exactly as before (turn chips, all turns, per-turn diffs)
• Toggle to git mode → verify working tree diff appears
• Make a manual file edit → verify git diff panel updates automatically (via status subscription)
• Run an agent turn → verify git diff updates as changes are written
• Toggle back to session mode → verify turn diffs are still intact
• Test on a repo with no commits → verify no crash (cached diff fallback)
• Test stacked/split and word-wrap toggles work in both modes
• Verify diffScope=git persists in URL across navigation

Note

Add git working tree diff toggle to the diff panel

• Adds a scope toggle (session / git) to the diff panel in DiffPanel.tsx, letting users switch between checkpoint diffs and the current working tree diff.
• The working tree diff is fetched via a new git.workingTreeDiff WebSocket RPC endpoint, backed by readWorkingTreeDiff in GitCore.ts, which runs git diff HEAD or git diff --cached depending on whether commits exist.
• Output is truncated at 512 KB (WORKING_TREE_DIFF_MAX_OUTPUT_BYTES). The diff is automatically re-fetched on git status changes.
• The selected scope is encoded in the URL via a new diffScope search param, parsed and validated in diffRouteSearch.ts.

^{Macroscope summarized a17b919.}

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Approvability

Verdict: Needs human review

This PR introduces a new user-facing feature: a toggle in the diff panel allowing users to switch between session-based diffs and git working tree diffs. While well-structured and self-contained, new features with user-facing behavior changes warrant human review. The author is also new to the primary files being modified.

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