fix(app): trust server session diff for Files Changed panel

[jeonghun-jj-lee]

Problem

The "Files Changed" panel showed every file the agent touched during a session, even when those files were reverted to their session-start state. Editing a file and then undoing that edit still left it in the panel.

Root cause

The reviewDiffs memo in session.tsx checked if (serverDiffs.length > 0) to decide whether to use the server's authoritative session-scoped diff. When the server correctly returned [] (all agent edits were reverted — no net changes), the condition was false and the code fell through to accumulateDiffs(), which counts tool operations rather than net filesystem state. Two edits to the same file (one forward, one back) always produced entries.

The same fallthrough happened during the placeholder phase of the TanStack Query, so the server path was effectively dead — the client always used the operation-based accumulator.

Fix

Extract resolveReviewDiffs — a pure, tested function that uses sessionDiffQuery.isPlaceholderData to distinguish "server responded with empty" from "query still loading":

• serverReady = true, diffs empty → return [] (the fix — trust the server)
• serverReady = true, diffs present → return normalized server diffs
• serverReady = false → fall back to accumulateDiffs so the panel shows immediate results during initial load

The server's Session.diff() (already in the pinned binary since v1.18.10-amicode.18) computes the net diff by comparing the first step-start git tree snapshot to the current working tree, filtered to agent-touched files. Files with zero net change are absent from its result.

Testing

• 8 new tests for resolveReviewDiffs covering: server-empty-means-empty, server diffs used when present, fallback while loading, path normalization, filtering
• All 2113 existing extension tests pass
• Full build succeeds

Files changed

File	Change
resolve-review-diffs.ts	New pure function
resolve-review-diffs.test.ts	8 tests
session.tsx	Wire resolveReviewDiffs into the reviewDiffs memo