Unified workspace layout: merge Grid and Dispatch into one stage-over-fleet workspace #992

Problem

Two layout modes (Grid tile trees, Dispatch) with two placement authorities, two focus truths, two spawn/close paths, and mode-split command surfaces (grid / dispatch). New users must pick a mode before they can work; command titles are mode-relative; every layout bug class on record came from one system mirroring the other.

Evidence: owner's live workspace runs 12 lanes in 2 rows, 14 detached sessions, and project tabs each holding a single pane — tabs are used as project buckets, not split trees. Nested splits are never used.

Proposal

One layout, per docs/superpowers/plans/2026-09-17-unified-stage-layout.md:

• Fleet pool is the only home for sessions (agents, terminals, extension views); "detached" becomes the default state.
• Stage = ragged rows of lanes (today's Grid Dispatch promoted from mode to workspace). Lanes are space; only the user names occupants; two sanctioned continuity writes (entry seed, context-places spawn).
• Projects are labels, not layouts — chips, not tabs; own no placement.
• One focus truth (focusedLane); Spotlight/Reader/Focus stay as lenses.
• Vocabulary: "Dispatch"/mode names retire from all user-facing language; grid+dispatch surfaces merge into one workspace surface; command ids stay stable so bindings never orphan.
• Onboarding: first run is one row x one lane, focused; Neovim-style starter card (registry-driven live keybinds) on fresh agents and empty lanes.

Deletes: TileNode/treeOps/TileTree splits, detachedSessions + buried buckets, mode toggle + switch-back banner, scope, Tile Tabs, normalize/rotate/nav commands.

Migration: workspace.json v2 -> v3 read-time pure function (tabs -> projects, detached/buried folded into pool, dispatchMode.tiled -> stage, accepted loss: multi-pane tab arrangements are pooled, not reconstructed).

Execution: 8 stages in the plan, each leaving npm run check green.