feat(compression): flip in_place default to True (#38763) [2/2]

In-place compaction (single durable session id, non-destructive soft-archive)
becomes the default. Rotation is now the opt-out fallback via
compression.in_place: false.

Prerequisite: #50098 (hygiene guard reads result flag not config flag) merged
first — without it, flipping the default causes permanent transcript loss on
gateway hygiene-compress and /compress when no session_db is available.

Blast radius (empirically measured on current main): 7 rotation-asserting
tests broke and are pinned to in_place=False in the companion test commit:
- tests/agent/test_compression_concurrent_fork.py (2)
- tests/agent/test_compression_logging_session_context.py (1)
- tests/agent/test_compression_rotation_state.py (1)
- tests/run_agent/test_compression_boundary_hook.py (2 _make_agent helpers)
- tests/gateway/test_compression_concurrent_sessions.py (2)
Rotation stays as a working fallback and deserves continued coverage.

Plan: .hermes/plans/in-place-compaction-38763.md

hermes_cli/config.py

@@ -1315,7 +1315,7 @@ DEFAULT_CONFIG = {
                                       # exact route is affected — gpt-5.5 on OpenAI's
                                       # direct API, OpenRouter, and Copilot keep the
                                       # global threshold regardless.
-        "in_place": False,            # When True, compaction rewrites the message
+        "in_place": True,             # When True, compaction rewrites the message
                                       # list and rebuilds the system prompt WITHOUT
                                       # rotating the session id — the conversation
                                       # keeps one durable id for its whole life