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_loop_consolidation_tokens.py

@@ -190,7 +190,8 @@ async def test_consolidation_persists_summary_for_next_prepare_session(tmp_path,
     reloaded, pending = loop.auto_compact.prepare_session(reloaded, "cli:test")
     assert pending is not None
     assert "User discussed project status." in pending
-    assert "_last_summary" not in reloaded.metadata
+    # _last_summary persists for restart survival.
+    assert "_last_summary" in reloaded.metadata
 
 
 @pytest.mark.asyncio
@@ -207,7 +208,6 @@ async def test_preflight_consolidation_receives_pending_summary(tmp_path) -> Non
 
     loop.consolidator.maybe_consolidate_by_tokens.assert_any_await(
         session,
-        session_summary="Previous conversation summary: earlier context",
         replay_max_messages=loop._max_messages,
     )