perf(compressor): release allocator pages after successful compaction

A successful compaction frees the largest allocation a long session ever
drops (the compressed-away message dicts), but Python's arena allocator
keeps those pages in the heap — RSS retains the pre-compaction
high-water mark until exit. #76905's trim_memory lifecycle covers the
gateway/TUI housekeeping loops but not the CLI compression path.

Call trim_memory(reason='post-compression') at the compression-success
point in ContextCompressor.compress(), following the house pattern
(lazy import in try, debug-level log on failure). The helper is
glibc-gated, config-gated and rate-limited, so it is a safe no-op on
other platforms and cannot fail compression.

Re-expresses the intent of #70782 (JonthanaHanh), which reached for a
bare gc.collect(); trim_memory is the house mechanism and already
wraps a collect.

agent/context_compressor.py

@@ -6730,6 +6730,26 @@ This compaction should PRIORITISE preserving all information related to the focu
         _strip_persistence_markers(compressed)
         self._last_compression_made_progress = True
 
+        # A successful compaction just freed the largest allocation a long
+        # session ever drops (the compressed-away message dicts), which makes
+        # this the natural point to hand allocator pages back to the OS.
+        # #76905's trim lifecycle covers the gateway/TUI housekeeping loops but
+        # not the CLI compression path, so RSS keeps the pre-compaction
+        # high-water mark until exit. The helper is glibc-gated, config-gated
+        # and rate-limited, so this is a safe no-op elsewhere. (#70782)
+        try:
+            from hermes_cli.mem_trim import trim_memory
+
+            trim_memory(reason="post-compression")
+        except Exception as exc:
+            # debug, not warning: sibling trim sites all log failures at
+            # debug, and compression must never fail because of a trim.
+            logger.debug(
+                "post-compression memory trim failed: %s: %s",
+                type(exc).__name__,
+                exc,
+            )
+
         # Batch compaction invalidates micro-compaction state: the batch
         # marker now holds MORE history than the in-memory rolling summary
         # (it summarized everything in the window, including exchanges micro

tests/agent/test_post_compression_trim.py

@@ -0,0 +1,66 @@
+"""A successful compaction hands allocator pages back to the OS.
+
+The compressed-away message dicts are the largest allocation a long session
+ever frees, but Python's arena allocator keeps those pages in the process heap
+— RSS retains the pre-compaction high-water mark until exit. #76905's
+trim_memory lifecycle covers the gateway/TUI housekeeping loops but not the
+CLI compression path, so compress() now calls
+``trim_memory(reason="post-compression")`` after a successful pass.
+
+trim_memory itself is glibc/Linux-gated (a fast no-op on macOS), so these
+tests monkeypatch the seam rather than asserting on RSS. Salvaged in spirit
+from #70782 (which reached for a bare gc.collect(); trim_memory is the
+house mechanism and already wraps a collect).
+"""
+import hermes_cli.mem_trim as mem_trim
+from agent.context_compressor import ContextCompressor
+
+
+def _compressor(threshold_tokens: int = 24_576) -> ContextCompressor:
+    cc = ContextCompressor(
+        model="test-model",
+        threshold_percent=0.75,
+        protect_first_n=5,
+        protect_last_n=20,
+        quiet_mode=True,
+        config_context_length=40960,
+        provider="test",
+    )
+    cc.threshold_tokens = threshold_tokens  # pin; don't couple to window math
+    cc._generate_summary = lambda *a, **k: "Summary of earlier turns."
+    return cc
+
+
+def _messages(n: int, size: int = 1500) -> list:
+    msgs = [{"role": "system", "content": "sys"}]
+    for i in range(n):
+        role = "user" if i % 2 == 0 else "assistant"
+        msgs.append({"role": role, "content": f"m{i} " + "z" * size})
+    return msgs
+
+
+def test_successful_compression_trims_memory_once(monkeypatch):
+    calls = []
+    monkeypatch.setattr(
+        mem_trim, "trim_memory", lambda *a, **kw: calls.append(kw) or False
+    )
+
+    cc = _compressor()
+    out = cc.compress(_messages(14), current_tokens=100_000)
+
+    assert len(out) < 15, "sanity: compaction should have made progress"
+    assert len(calls) == 1, "trim_memory must run exactly once per compaction"
+    assert calls[0].get("reason") == "post-compression"
+
+
+def test_trim_failure_does_not_break_compression(monkeypatch):
+    def boom(*a, **kw):
+        raise RuntimeError("allocator says no")
+
+    monkeypatch.setattr(mem_trim, "trim_memory", boom)
+
+    cc = _compressor()
+    out = cc.compress(_messages(14), current_tokens=100_000)
+
+    assert cc._last_compression_made_progress is True
+    assert isinstance(out, list) and out, "compress() must still return messages"