File edit/diff tracking: per-turn rollup + tested shared diff util

[armstrongsamr]

Closes #80

Implemented autonomously by the Sandcastle harness from issue #80, then rebased onto current main (the agent's sandbox was based on a stale local checkout; cleaned to the 7 feature files only).

What this does

• Extracts the hand-rolled Myers/LCS diff out of the 3000-line ToolGroup.tsx into a pure, tested shared/edit-diff.ts (computeDiffLines / computeEditStat / buildEditSummary / isEditToolName).
• Adds a per-turn rollup footer (EditDiffSummary.tsx) under each assistant turn's tool group: "N files changed · +X −Y".
• New config ui.editDiffSummary.enabled (default true), wired into both the schema and the desktopConfigPayload() allowlist.

Host-side validation (macOS)

• pnpm type-check (tsc --noEmit): clean
• electron/__tests__/edit-diff.test.ts: 34 passed
• src/components/thread/__tests__/EditDiffSummary.component.test.tsx: 12 passed

Security

Design-level Cerberus pass (APPROVE) folded its hardening into the issue ACs; covered here: diff rendered as React text children only (no dangerouslySetInnerHTML — XSS-literal-text test included), non-quadratic rollup, truncation flag for >400-line block fallback, /+\ separator-safe basename.

Screenshot of the feature (multi-file edit turn showing the rollup footer) to be added — captured host-side on macOS.

[github-actions]

Coverage report

Project-wide coverage (reporting only — no gate):

Metric	This PR	Baseline
lines	32.84%	—
statements	31.52%	—
functions	29.63%	—
branches	25.15%	—

No baseline available — first run, missing artifact, or fetch failed.

Changed-file coverage (3 of 5 files instrumented; 2 not in unit slice)

Per-file table

File	Lines	Statements	Functions	Branches
electron/config/schema.ts	100.00%	100.00%	100.00%	100.00%
electron/ipc/config.ts	43.89%	42.30%	52.94%	30.29%
shared/edit-diff.ts	97.70%	98.03%	100.00%	97.46%

Coverage is reporting-only. No thresholds, no merge gate.

[armstrongsamr]

Review: please split this PR

The in-scope #80 feature is clean and would be approvable on its own — shared/edit-diff.ts (pure module mirroring shared/token-usage.ts, DP allocation gated by BLOCK_DIFF_THRESHOLD, separator-safe baseFileName never handed to fs/path), EditDiffSummary.tsx (renders only numeric counts as JSX text — the component test asserts <img onerror=…>/</script> are never reflected), the ToolGroup refactor, the schema/allowlist change, and ~full module coverage.

What's blocking is everything else that rode along (all out of #80 scope):

1. Scope creep — ~85 files vs the ~6 the issue scopes. Includes evals workflows + 18 baseline JSONs, a fixture-drift workflow + script, the integration-real suite, a vitest config split, a sub-agent cleanup service + IPC, a runtime-compat module, step-tracking UI, AdvancedSettings.tsx, a maxSteps 10→25 default bump, issue templates, CHANGELOG/MAINTAINERS/TESTING_ARCHITECTURE. None of it is in the issue's implementation plan.
2. A mocked backend shipped as real UI. AdvancedSettings.tsx uses setTimeout(…,500) to return hardcoded zeroed stats (comment: "we'll simulate it"), then renders a "Recommendation" panel + an "Apply Recommendation" button that writes advanced.maxSteps from fabricated data.
3. New privileged CI automation. evals.yml adds peter-evans/create-pull-request (tag-pinned, not SHA-pinned) auto-pushing branches with GITHUB_TOKEN; several workflows pin actions to floating tags. Third-party actions should be SHA-pinned, and a PR-writing bot deserves its own review.
4. An unrelated sensitive change. electron/ipc/agent.ts's plugin-runtime fail path interpolates a config-controllable pluginRuntimeId into a user-visible error string.
5. Minor: the edit-diff.test.ts boundary test passes on either branch (doesn't pin the threshold); buildEditSummary's per-file last-write-wins is iteration-order dependent.

Recommendation: land #80 as just the diff util + EditDiffSummary + ToolGroup refactor + schema/tests; move the evals/CI, sub-agent cleanup, and settings work into their own PRs so each is reviewed on its merits. Marking draft pending the split.