fix(agent): move archived summary into system prompt for KV cache stability

- Append [Archived Context Summary] to system prompt instead of injecting
  it into the user message runtime context, improving KV cache reuse across
  turns and avoiding consecutive same-role messages.
- _last_summary persists in metadata (no pop) for restart survival;
  summary is re-injected every turn via the stable system prompt.
- Remove dynamic "Inactive for X minutes" from _format_summary — use
  static last_active timestamp instead to preserve KV cache stability.
- Pass session_summary through build_messages() so both normal and
  ask_user paths receive the archived summary in the system prompt.
- estimate_session_prompt_tokens now reads _last_summary from metadata
  to include the summary in token budget estimation.
- Remove obsolete session_summary parameter from
  maybe_consolidate_by_tokens and estimate_session_prompt_tokens
  call sites in loop.py (summary flows through build_messages instead).
- Ensure /new (session.clear()) clears _last_summary from metadata.

tests/agent/test_unified_session.py

@@ -399,7 +399,6 @@ class TestConsolidationUnaffectedByUnifiedSession:
         # estimate was called (consolidation was attempted)
         consolidator.estimate_session_prompt_tokens.assert_called_once_with(
             session,
-            session_summary=None,
         )
         # but archive was not called (no valid boundary)
         consolidator.archive.assert_not_called()