feat!: adopt atelier's single central-tabs primitive

[samuelstroschein]

Adopts atelier's new single central-panel primitive (opral/atelier#60): the central panel is now always browser-style tabs. Per the final API shape, hosts that configure no centralPanel.home — FlashType — keep the Files view in the left sidebar (open by default on fresh state): documents open as central tabs, there is no pinned center tab, and closing the last tab shows the central empty state ("Start writing").

Important

BLOCKED on opral/atelier#60 merging. The submodule/atelier pointer targets commit 207d18e8d84f4e1bf6c0ad8fdf5259840527a3c5 on the central-panel-embedding branch. The CI step "Validate submodule branches" requires the pinned sha to be reachable from atelier origin/main, so it will fail until atelier#60 merges. After the merge, bump the pointer to the merge commit and this PR becomes mergeable.

The branch is based on the current main (aba390a) — main has not moved since, so no rebase was needed; the file-picker/strip UX parity work came in atelier-side via the submodule bumps.

Behavior changes (UI-visible)

• Documents now open as browser-style tabs in the central panel (previously a single document slot). The Files sidebar, central empty state, and draft flow stay as they were.
• The Files tree is now atelier's bundled view (colored file-type glyphs, virtualized tree, active-document auto-reveal) instead of FlashType's fork — see "Files tree consolidation" below.
• The checkpoint feature is removed entirely, mirroring upstream atelier's removal (Remove checkpoint implementation atelier#59, pulled in by this submodule bump): the History panel (checkpoint list, create/rename/switch/delete), agent-generated checkpoint names, checkpoint-diff viewing, and checkpoint file markings are all gone. The shared lix branch plumbing (createBranch/switchBranch/syncDiskToLix client APIs, the lix_workspace_branch_id active-branch key used by agent review and editors) stays.

What changed

Central tabs adoption

• submodule/atelier: c5d8a35 → 207d18e (central-panel-embedding tip: always-tabs central panel; pinned home only when configured; Files sidebar-only; strip UX parity; host watched-files data source).
• src/main.tsx: dropped filesViewMode: "sidebar" — the option is fully derived by atelier now.
• pnpm-lock.yaml: additive update for the new atelier deps (excalidraw, mermaid, etc.).

Files tree consolidation

• Deleted the ~5,450-line files-extension fork (src/extensions/files/: view, file tree, tree builder, host extension, tests) plus the now-dead src/queries.ts and the fork-only ExtensionContext/WorkspaceContext/PanelSide types. Atelier's bundled Files extension takes over (hosts that register no replacement get it by default). Net delta of the consolidation commit: +110 / −5,679 lines.
• Review overlays are native: the bundled view computes pending external-write-review paths itself (agent-review e2e passes unchanged). FlashType doesn't tag review ranges with session ids, so no reviewRangeSessionId wiring was needed.
• Deletion-closes-tab is native: the bundled view wires closeFileViews → documents.close(path) itself, superseding the host-side fix from earlier in this PR.
• Transient-workspace watched files relocated host-side to src/lib/atelier-files-view.ts via atelier's new createAtelier({ filesView }) options (207d18e): watchEntries bridges workspace.setEphemeralWatchedDirectories({ownerId, paths: expandedDirectories}) + the onEphemeralWatchedFileTreeChanged subscription, and resolveFileForInteraction does the lix lookup → importFilesystemPaths → canonical-id import on first open/rename. Only ephemeral workspaces pass filesView.
• e2e: one adjustment — the bundled tree auto-reveals (expands) the active document's directory, so the grouped-transient-workspace test no longer clicks that directory (the click would collapse it); all other selectors held (data-item-* attributes are a superset of the fork's).

Checkpoint removal (mirrors atelier#59)

• Deleted: src/extensions/history/ (the checkpoint UI), electron/checkpoint-name.mjs (+test, IPC handler, preload method, types), src/shell/checkpoint-diff.ts (+test), src/extension-runtime/checkpoint-diff.ts, e2e/branch-switcher.spec.ts (all four tests exercised checkpoint UI/diffs) and the history e2e helper.
• Stripped checkpoint branches from: the legacy MarkdownView/CsvView (checkpoint-diff rendering; commit-snapshot/diff modes stay — they back agent-review rendering), the top bar (isReviewingCheckpoint badge), editor-revision-state (checkpoint state keys).
• Kept deliberately: lix branch plumbing (createBranch/switchBranch/syncDiskToLix, the lix_workspace_branch_id active-branch key) — agent review, the tip-tap editor, and workspace machinery read them; agent-review machinery (external-write-review*, agent-turn-review-range) is a separate feature and untouched.

Verification

• pnpm run typecheck — clean
• pnpm run test — 461 passed, 1 skipped (fork + checkpoint tests deleted)
• pnpm run build (incl. atelier submodule build) — clean
• pnpm run lint / prettier — only pre-existing findings
• Electron e2e run locally headless at the final shape: agent-review (review overlay renders on the bundled tree), files-click-tour 2/2, workspace-switch, headless-launch, workspace-windows 16/16 — including the transient-workspace tests that surface watched disk files through the new filesView bridge

After atelier#60 merges

cd submodule/atelier && git fetch origin main && git checkout <merge-commit>, commit the pointer bump; the submodule-branch validation then passes.

🤖 Generated with Claude Code

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e2e/ packaged-macos.spec.ts:101:1 › packaged app launches, seeds, and opens files without Vi te ─	View Logs