feat(runtime): add bounded foreground ad hoc child route

[testikun]

Summary

Implements the bounded foreground ad hoc child-agent slice for #4538:

• adds a desktop setting for an explicitly enabled temporary child-agent policy, including the maximum profile and model connection/model/thinking-level selection;
• exposes the synthetic temporary-bounded route through the existing agent_list/agent_spawn catalog path only when enabled and its host-resolved model connection is available;
• preserves an existing ad hoc policy when ordinary presets are updated;
• accepts only bounded task-scoped role text, while Runtime/Host continue to select the profile, model, tools, permission, workspace, lifecycle, and nesting authority;
• rejects model-supplied overreach (model, connection, tools, permission, workspace, lifecycle/nesting, write-back/isolation, and a second profile selector);
• rejects invalid role data, nested creation, graph/background use, unavailable routes, and over-broad contract requests before child Session creation;
• rejects child creation from Side Conversations as well as linked child Sessions;
• preflights the temporary child against the parent Session execution boundary;
• preserves the existing linked Session/RuntimeInvocation lineage, idempotency, cancellation, recovery, transcript isolation, and artifact paths;
• extracts shared synthetic-preset construction and adds focused coverage.

Current-head real App verification (2026-09-04)

Built and launched pre-rebase feature tree 58979f3 in a visible local Electron window; current head 7b81677 is the rebased equivalent on apache/main 19ac204ab, opened Settings → Subagents, enabled the temporary-subagent policy, saved it, and verified the persisted Host setting. These are direct screenshots from the running Maka App; they are not mockups or generated images.

1. Current-head disabled state with the new route controls

2. Current-head enabled state after saving the policy

Verification

• rebased the current PR branch onto current apache/main (19ac204ab)
• full workspace and Desktop production build — passed
• Desktop typecheck — passed
• renderer architecture checker — passed (71/71)
• child-session Runtime test group — passed (16/16)
• real Electron persistence journey: disabled → enable → save → persisted Host setting — passed
• Biome check on all 19 changed TypeScript, TSX, and CSS files — passed
• git diff --check — passed
• legacy screenshot binaries removed from the source diff; evidence remains in this first PR description

AI use

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: OpenAI Codex provided material implementation and review assistance, fixed the architecture-budget and RuntimeInvocation convergence regressions, added a real Electron persistence test, ran visible App verification, and prepared the commits.

Closes #4538