feat(domain,infra,app): borne summary_md du handoff + durcissement seam LLM (LS5)

Clôt le must-have perf du handoff :
- domain : HANDOFF_SUMMARY_MAX_CHARS + bound_handoff_summary (helpers privés), re-export lib.
- infrastructure : TURN_LINE_MAX_CHARS + élision dans render_turn (summarizer), re-exports.
- application : borne entre fold et save (conversation/record), borne défensive
  dans resolve_handoff (agent/lifecycle) + module test.
- seam LLM prêt mais NON activé (aucun LLM câblé).
- tests (QA, verts) : conversation_log, conversation_record, agent_lifecycle.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/infrastructure/src/lib.rs

@@ -39,7 +39,8 @@ pub mod timeparse;
 pub use clock::SystemClock;
 pub use conversation::InMemoryConversationRegistry;
 pub use conversation_log::{
-    FsConversationLog, FsHandoffStore, FsProviderSessionStore, HeuristicHandoffSummarizer, WINDOW,
+    FsConversationLog, FsHandoffStore, FsProviderSessionStore, HeuristicHandoffSummarizer,
+    TURN_LINE_MAX_CHARS, WINDOW,
 };
 pub use eventbus::TokioBroadcastEventBus;
 pub use fileguard::RwFileGuard;