refactor(dashboard): remove the legacy layout engine (dockview-only)

[aterrylu]

Summary

Removes the legacy layout engine now that dockview is the default (ADR-047 "delete legacy" phase; follows #263). This is the atomic deletion the ADR planned: ~3,500 LOC of fragile hand-rolled layout code comes out, leaving dockview as the only engine.

No user-facing behavior change — dockview already shipped on by default in #263 and is untouched here. This PR is pure subtraction + two small reconciliation fixes.

What's deleted

graph LR
  subgraph Gone["Deleted (~3.5k LOC)"]
    direction TB
    A["layoutTree.ts (+723-line test)"]
    B["SessionMountLayer<br/>(detached overlay)"]
    C["SplitLayout · PaneSlot<br/>DropZoneOverlay · TabBar · LayoutContext"]
    D["groups workspace-swap<br/>+ 11 split-pane store actions"]
    E["layoutEngine flag<br/>+ dual-engine branches"]
    F["react-resizable-panels dep"]
  end
  subgraph Kept["Remaining engine"]
    G["dockview-react<br/>(the only layout engine)"]
  end
  Gone -.retired.-> G

Loading

• 8 files deleted: layoutTree.ts (+ its 723-line test), SplitLayout.tsx, PaneSlot.tsx, SessionMountLayer.tsx, LayoutContext.tsx, DropZoneOverlay.tsx, TabBar.tsx.
• store.ts: removed the layout/focusedLeafId/groups/activeGroupId state + 11 binary-tree/group actions + their persist entries; the layoutEngine flag is gone (dual-path actions collapse to their dockview body).
• DragContext.tsx: kept the drag serialization (DRAG_TYPE/encodeDragData/decodeDragData/DragData — dockview still uses these); removed the legacy React DragProvider/useDragContext.
• react-resizable-panels removed from dependencies.
• Legacy keyboard shortcuts (Ctrl+D / Ctrl+Shift+D / Ctrl+W) removed — they drove the legacy tree and were half-broken under dockview. dockview-native keybinds are a planned follow-up (Terry's call: drop now, port later).

Re-sourced: the sidebar's on-screen-agent indicator

The only behavioral wiring change. The sidebar marked which agent rows are currently on-screen via allTabPanes(layout) (the deleted tree). It now reads a new visiblePaneIds store field that DockviewLayout publishes from its live panel set (onDidLayoutChange + after restore/drop). Cleared on unmount so it doesn't go stale when the dock empties.

Review fixes

A code-reviewer pass on the diff caught two reconciliation gaps (both from removing the layout tree as the single reconcile source) — fixed:

1. Closed-preview orphan: closing a preview while another pane was active left a blank "Preview" tab (the prune only ran on sessions changes). The prune effect now also triggers on previewPanes.
2. Stale visiblePaneIds on unmount: killing the active pane left the sidebar highlighting a now-empty dock. visiblePaneIds is now cleared on DockviewLayout unmount.

Testing

• make check green: biome + tsc --build + 613 server tests + 227 dashboard tests.
• Two agent reviews: one mapped the full removal surface (what to delete vs preserve — caught that DragContext's serialization is still needed by dockview); one reviewed the diff and found the two reconciliation gaps above (fixed).
• Runtime gate = CI e2e (real Playwright). dockview's interactive behavior — click=solo, drag=compose, workspace restore, cold-reload, hidden-panel visibility, overlay suppression — was exhaustively Playwright-verified in feat(dashboard): rebuild tabs + split-pane layout on dockview, default on #263 and is unchanged here, so this PR leans on CI e2e rather than re-running that suite locally.

Risks

• Pure deletion + the layoutEngine flag is gone, so there's no runtime fallback to legacy anymore — intentional (ADR-047). The legacy code is fully removed, not just default-off.
• Rebased onto latest main (includes fix(dashboard): clearer permission-mode labels (Ask, Accept edits) #265 permission-mode labels) so the diff is legacy-only.

🤖 Generated with Claude Code

https://claude.ai/code/session_01MGMW4cbhv5Le8wka7Th8bS

[nox-0x]

Approving — clean atomic deletion of the legacy layout engine; the two reconciliation fixes (prune effect now triggers on previewPanes change; visiblePaneIds cleared on DockviewLayout unmount) close the obvious gaps a tree-as-source-of-truth removal would have left. Sound reasoning throughout:

• The visiblePaneIds round-trip (DockviewLayout's onDidLayoutChange → store → Sidebar memo'd Set) replaces allTabPanes(layout) cleanly. pushVisible is stable ([] deps) and fires on all add/remove/move events.
• CreateAgentPanel no longer self-closes — verified spawnSession calls switchPane({type:'session',id}) on success (store.ts:574), which solo-replaces the create-agent panel via syncToActive.
• Effect ordering safe: syncToActive (mount-restore) runs before pruneDead (declaration order), so newly-added previews/sessions never get pruned mid-add.
• Removing the legacy-only pruneStaleSessionTabs branch in fetchSessions is correct — dockview owns topology and pruneDead handles dead-session panels.
• Tests trimmed to the actual surface (switchPane semantics under dockview); legacy split/move/closeTab suites correctly gone with the code they exercised.

Minor follow-up (non-blocking): README.md:109 still lists react-resizable-panels in the tech-stack line — the dep is gone now, swap to dockview-react whenever convenient.