feat: make changes panel Git-authoritative

[Astro-Han]

Summary

• make the desktop Changes panel read only the live current-branch Git comparison
• remove the session-message-derived Last turn source and the non-Git fallback
• remove scope, base-branch, refresh, staging, and revert controls from the Changes panel
• replace the opaque vs main · 3 files · +63 · -98 metadata line with explicit changed-file, added-line, and deleted-line labels
• keep every file diff collapsed initially and allow at most one on-demand expansion in the compact workbar
• align the layout with the Astryx IDE inspector pattern: 16px panel gutter, quiet summary, one inset grouped file list, and an inset diff surface
• hide zero-value summary rows in the empty state and preserve file statistics when long paths truncate
• compose the surface from Astryx Section, Stack, Text, Collapsible, EmptyState, Banner, and Button primitives; retain Maka's diff renderer because Astryx has no diff primitive
• add a real Session Workbar Storybook story for the Git-backed Changes panel
• keep Git Review mutation APIs, ArtifactStore, and tool-result persistence unchanged outside this read-only surface

Verification

• npm --workspace @maka/desktop run typecheck
• npm --workspace @maka/desktop run test:checks
• npm --workspace @maka/desktop run build:main
• node --test apps/desktop/dist/main/__tests__/conversation-review-copy.test.js
• npm --workspace @maka/desktop run build-storybook
• node scripts/check-astryx-alignment.mjs
• node scripts/check-astryx-surface-inventory.mjs
• focused Biome correctness lint
• git diff --check
• visual verification in Storybook for collapsed and expanded states
• visual verification in the real Electron product for empty, collapsed, and expanded states

Desktop sources are intentionally excluded from the repository formatter pending #415; the applicable Biome lint gate passed.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No