refactor(compression): tidy in-place compaction path (simplify pass)

Parallel 3-reviewer cleanup of the in-place compaction code. Findings applied:

- perf: in-place mode no longer pre-flushes current-turn messages. The flush
  ran INSERTs that the immediately-following replace_messages(compressed)
  DELETE+reinsert discarded -- pure wasted writes per compaction. The
  current-turn tail survives via the compressor's compressed output
  (protect_last_n), not the flush. Verified no data loss; rotation still
  pre-flushes (its old session row is preserved, so the flush is real there).
- quality: hoist the two shared post-write steps (update_system_prompt +
  _last_flushed_db_idx = 0) below the if/else -- they ran in both branches
  against agent.session_id. Removes the easiest divergence bug.
- quality: compute the compaction-boundary locals (_old_sid, _is_boundary,
  _boundary_parent) ONCE instead of recomputing locals().get('old_session_id')
  and the "_old_sid or agent.session_id or ''" chain three times.
- quality: initialize compacted_in_place up front and assign
  agent._last_compaction_in_place directly, dropping the fragile
  locals().get('compacted_in_place') reflection.
- reuse: parse the in_place config flag with utils.is_truthy_value (the
  project's canonical truthy coerce) instead of a hand-rolled
  str().lower() in {...} (agent_init already imports from utils).

Dropped as false positives / out of scope: gateway getattr of agent internals
(established session_id pattern), dual result-dict carry (mirrors history_offset
etc.), stringly-typed "compression" (codebase-wide convention, no constant).

Behavior-preserving: 7 in-place tests (incl. 2 new flush-guard tests) + 26
rotation/boundary/persistence/command tests green; mutation check confirms the
durable-replace guard still binds (removing replace_messages fails the test);
ruff clean. Added test_in_place_skips_redundant_preflush /
test_rotation_still_preflushes to guard the perf change.

tests/run_agent/test_in_place_compaction.py

@@ -124,6 +124,48 @@ class TestInPlaceCompaction:
             roles = [m["role"] for m in compressed if m.get("role") != "system"]
             assert all(roles[i] != roles[i + 1] for i in range(len(roles) - 1))
 
+    def test_in_place_skips_redundant_preflush(self):
+        """In-place must NOT pre-flush current-turn messages: replace_messages
+        rewrites the whole row, so a flush would INSERT rows it immediately
+        deletes (wasted writes). The current-turn tail survives via the
+        compressor's `compressed` output, not the flush."""
+        from hermes_state import SessionDB
+        from agent.conversation_compression import compress_context
+
+        with tempfile.TemporaryDirectory() as tmp:
+            db = SessionDB(db_path=Path(tmp) / "t.db")
+            _seed(db, "ip_flush", "f")
+            agent = _make_agent(db, "ip_flush", in_place=True)
+            calls = {"n": 0}
+            agent._flush_messages_to_session_db = lambda *a, **k: calls.__setitem__(
+                "n", calls["n"] + 1
+            )
+            compress_context(
+                agent, [{"role": "user", "content": "x"}] * 8,
+                approx_tokens=100_000, system_message="sys",
+            )
+            assert calls["n"] == 0
+
+    def test_rotation_still_preflushes(self):
+        """Rotation MUST pre-flush so current-turn messages survive in the
+        preserved old (parent) session before it is ended (#47202)."""
+        from hermes_state import SessionDB
+        from agent.conversation_compression import compress_context
+
+        with tempfile.TemporaryDirectory() as tmp:
+            db = SessionDB(db_path=Path(tmp) / "t.db")
+            _seed(db, "rot_flush", "f")
+            agent = _make_agent(db, "rot_flush", in_place=False)
+            calls = {"n": 0}
+            agent._flush_messages_to_session_db = lambda *a, **k: calls.__setitem__(
+                "n", calls["n"] + 1
+            )
+            compress_context(
+                agent, [{"role": "user", "content": "x"}] * 8,
+                approx_tokens=100_000, system_message="sys",
+            )
+            assert calls["n"] == 1
+
 
 class TestRotationStillDefault:
     def test_rotation_when_flag_off(self):