[Bugfix #1182] Group idle sibling architects under 'Idle Architects' node (main always separate)

[amrmelsayed]

Fixes #1182

Summary

Follow-up to #1174 / PR #1175. Now that childless architects stay visible under the architect axis of the Agents view, a workspace running several quiet siblings (reviewer, demos, ide, mobile, …) accumulates 3-5 childless architect rows that displace real builder work below the fold. This groups idle sibling architects (zero builders, never main) under a single collapsed "Idle Architects (N)" container — but only when ≥ 2 are idle. Scope is architect-axis only; stage and area axes are unchanged.

Behavior:

Idle sibling count	Rendering
0	No container, no visual change
1	The lone idle sibling keeps its own top-level (0) row (a container would save no space for one row)
≥ 2	All idle siblings fold under ▶ Idle Architects (N), collapsed by default

main is always its own row regardless of builder count — it is the always-present workspace home base (created by launchInstance on every workspace; siblings are opt-in via add-architect), so burying it behind a chevron is a bad trade for one row of space. Populated architects (main + siblings) render as their own top-level groups with builders expanded, unchanged from #1174. Ordering: main → populated siblings → idle container (bottom).

Root Cause

packages/vscode/src/views/builders.ts rootChildren() mapped every architect group (including the childless ones #1174 made visible) to its own top-level header. With N idle siblings that is N rows of chevron-less (0) headers, each ~22px, pushing actual builder rows off-screen — the proportion cost #1174 traded for never losing an architect row mid-session.

Fix

• builder-tree-item.ts: new IdleArchitectsGroupTreeItem — label Idle Architects (N), Collapsed by default, a stable id (idle-architects-group) so VSCode persists expansion, contextValue group-idle-architects, the same neutral circle-outline/disabledForeground idle glyph a childless architect carries, and no command bound (a header click only toggles — there is no single terminal for a container of N architects).
• builders.ts: extracted the inline group-row builder into makeGroupRow(g, now, isArchitectAxis) (unchanged rendering: None for childless, Expanded for populated, openArchitectTerminal command on architect headers). New architectRootChildren() partitions groups into main (always own row), populated siblings (own rows), and idle siblings — emitting the container only when idle ≥ 2, else rendering the lone idle sibling as its own row. New idleArchitectRows() recomputes the container's children on expansion (each a childless BuilderGroupTreeItem that still opens its own terminal), so a click on a name inside the group opens that architect's terminal. getChildren dispatches on the new item type.

No extension.ts change (the accordion's onDidExpandElement only acts on BuilderTreeItem; the container toggles natively). No package.json menu change (no menu when-clause targets a Builders-view group contextValue).

Net diff: ~145 LOC src (2 files), ~277 LOC tests.

Test Plan

pnpm check-types clean, pnpm lint clean, pnpm vitest run → 638 pass (54 files), node esbuild.js --production bundles.

• New builders-idle-architects-group.test.ts (11 cases): fresh workspace (main only, no container); 1 idle sibling → own row, no container; 2 idle siblings → collapsed Idle Architects (2); 4 idle / everyone idle; main-always-separate under every combo; ordering (main → populated → container); the container's neutral glyph + no-command + contextValue; expansion yields the individual architect rows each opening its terminal; both transitions (sibling gains a builder → graduates out; drops to 1 idle → container dissolves to a lone row); stage axis unaffected.
• Updated one vscode: architects disappear from Agents view (architect axis) when their last builder is cleaned up #1174 test in builders-childless-architect-header.test.ts that pinned the pre-vscode: group idle sibling architects under 'Idle Architects' collapsed node (main always separate) #1182 behavior (3 childless architects all top-level) to the lone-idle-sibling case — the ≥2 interaction is now covered by the new file.