feat(agentic): add explicit background subagent result waiting

[wsp1911]

Summary

Replace automatic background-subagent result delivery with explicit AgentWait collection.

• Add durable, parent-owned background task outcomes and the AgentWait tool.
• Make background Task calls return background_task_id; completed results no longer wake or inject into the parent session automatically.
• Remove the public allow_review_follow_up control and expose AgentWait in applicable agent modes and the Web UI delegation group.

Type and Areas

Type:

Feature / regression fix

Areas:

Rust core, agent runtime, web UI

Motivation / Impact

Automatic result delivery could wake a parent session after it had independently completed its work, while parents that depended on a result could appear to stop prematurely. Background work is now collected only when the parent explicitly calls AgentWait.

A background Task response now includes background_task_id. Agents whose final answer depends on background work must call AgentWait; otherwise they can finish without being reactivated by a late result.

Verification

• pnpm run fmt:rs passed.
• cargo check -p bitfun-core --tests -j 1 passed.
• Focused bitfun-core lib tests passed for agent_wait_tool, one-time outcome consumption, timeout retention, and Task schema removal of allow_review_follow_up.
• pnpm --dir src/web-ui run test:run src/app/scenes/agents/components/toolGroups.test.ts passed (6 tests).
• pnpm run type-check:web passed.
• git diff --cached --check passed.

Reviewer Notes

• AgentWait can target explicit task IDs or collect unconsumed background outcomes from the current parent turn.
• Outcomes are consumed only through AgentWait; a timeout leaves unfinished tasks available for a later wait.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above.
• User-facing strings, docs, and locales are updated where applicable.