feat(web): inspect bounded Subagent owner details

[testikun]

Problem

Subagent activity in the Web workbench is a passive chip. The manager already owns exact progress and terminal outcome, but only Background Terminals have an inspectable detail path. Refs #346.

Value

Operators can inspect a Web Session’s delegated work, including its model, source, turn/record counts, recent tools, bounded output, failure and interruption evidence, without inventing a second resource state or adding browser control authority.

Approach

• Extend the existing Session-scoped Web observer registry with an exact-id Subagent detail projection from manager.view.get(id); the existing authenticated /api/capabilities/detail Host route continues to enforce the active Session identity.
• Bound title/model, cwd, error, current output and the last eight tool previews at the projection boundary; report omitted bytes/tools and truncation. A restarted running child does not present a previous run’s final text as current output.
• Wire Subagent chips into the existing InspectionPanel as text-only, read-only details. Keep terminal inspection and runtime status separate; no stop/cancel/write API is introduced.
• Ablation removed an extra queued-message field; turn count, retained record count, recent tool progress and exact outcome still satisfy this slice.

Validation

• bun run check: passed (Node 24, checkout base f6b49ae59605b1276b8267f2886d22c03f01533c).
• bun run test: Node 1660 passed, 1 platform skip; Vitest 222 passed.
• Focused Host/registry 47/47 and UI 51/51 passed. Real Chrome Subagent desktop/mobile test passed, including refresh, exact Session/id GET, escaped text and no writes; isolated pi list showed one local OpenPI source at this checkout.
• Full real-Chrome Web suite: 33/34 passed. The existing touch-viewport test intermittently closed its sidebar after a page-level Escape and timed out looking for Workspace options; that unchanged test passed when rerun alone. PR feat(web): browse archived and terminal Session history #547 fixes this test focus race, so this PR intentionally does not duplicate its patch. The new Subagent test passed in both full and focused runs.

Impact

• User-visible: Subagent activity chips open a bounded, read-only detail panel; existing Terminal and status panels remain available.
• Model-visible context/tools: none.
• Runtime/lifecycle: one synchronous owner read through the existing registry; no new controller, cancellation, or persisted state.
• Persisted config/data: none.
• Compatibility/risk: output is intentionally partial; full child transcript stays in Pi. This branch shares generated Web assets and UI files with PR feat(web): browse archived and terminal Session history #547, so it should be rebased after that PR lands. Refs feat(web): add inspectable and controllable OpenPI capability panels #346, not a closure of Workflow/control work.