refactor(protocol): type sub-agent completion as MessageSource::AgentResult

[yishuiliunian]

Summary

• Promote sub-agent completion results from an in-body <agent-result name=...> string to a typed MessageSource::AgentResult { child } variant, so the semantic lives in the type system rather than being string-parsed/string-rendered by each consumer.
• The envelope body stays raw; the <agent-result> wrapper is rebuilt only at LLM projection time. Cross-hub completion detection now matches the typed source instead of byte-scanning the content.
• Sweeps two adjacent debts: SSOT the turn-trigger LLM-text prefixes into trigger_llm_text (shared by wire projection + Anthropic builder), and fix DegenerationDetector to reuse MessageSource::is_task_boundary (its hand-rolled list silently dropped peer-agent envelopes its comment claimed to clear).

Changes

• loopal-protocol: new MessageSource::AgentResult { child } variant + label/is_task_boundary/prepend_hub/prepend_hub_if_local arms.
• loopal-turn: new TurnTrigger::AgentResult variant.
• loopal-agent-hub: producers (completion.rs/finish.rs) emit raw body + typed source; uplink.rs detects completion by source type (removed extract_agent_result_name byte-parser).
• loopal-provider-api: extract trigger_llm_text SSOT (prefix.rs); refactor trigger.rs into text + origin projection.
• loopal-provider: Anthropic trigger_user collapses onto the shared trigger_llm_text.
• loopal-runtime: turn_trigger_map maps AgentResult source (bare child name); degeneration_detector reuses is_task_boundary.
• loopal-tui: render_inbox_origin shows a ✅ result from <child> badge instead of raw XML tags.
• Tests updated/added across protocol, provider-api, runtime, tui, meta-hub.

Notes

• The <agent-result name=...> marker uses the bare child name even for cross-hub completions (hub path stripped) to match the prior contract.
• Agent completions deliberately drop the redundant [from: <child>] prefix — the marker already identifies the source.
• ACP _loopal/inbox.enqueued.source now emits the new AgentResult wire variant (serde pass-through); IDE clients should handle it.

Test plan

• CI passes (bazel test //..., clippy, rustfmt)