fix(compress): classify unconfirmed lock-acquire failures and cover all manual-compress surfaces

Follow-up to the salvaged #57634 commits:

- agent/manual_compression_feedback.py: new describe_compression_lock_skip()
  — single source of truth for lock-skip wording. A descriptive holder
  string means another compressor CONFIRMED holds the lock ('already in
  progress (holder: ...)'); True/None means acquisition failed without a
  confirmed holder (hermes_state.try_acquire_compression_lock catches
  sqlite3.Error internally and returns False), so the message says
  'could not acquire ... the lock check failed' instead of falsely
  claiming a concurrent compression is running.
- cli.py, gateway/slash_commands.py, tui_gateway/server.py (all three
  in-process consumers: session.compress RPC, command.dispatch compress
  branch, slash.exec mirror) now route through the shared helper.
- tui_gateway/server.py command.dispatch compress branch: catch
  CompressionLockHeld explicitly — it previously fell into the generic
  'compress failed' error handler.
- Deferred-notify contract (#69324): lock-skip discards the pending
  context-engine notification (committed=False) in _compress_session_history
  and the CLI path before returning.
- tests: lock-skip wording pins per surface, VISIBLE_COMPRESSION_MESSAGES
  noise-filter carve-outs for both wordings, MagicMock signal opt-outs for
  sibling tests added on main after the original PR.

agent/manual_compression_feedback.py

@@ -7,6 +7,36 @@ from typing import Any, Sequence
 from agent.redact import redact_sensitive_text
 
 
+def describe_compression_lock_skip(lock_signal: Any) -> str:
+    """User-facing text for a manual /compress skipped by the compression lock.
+
+    ``lock_signal`` is ``agent._compression_skipped_due_to_lock`` (or the
+    ``holder`` carried by the TUI's ``CompressionLockHeld``): a descriptive
+    holder string when another compressor CONFIRMED holds the lock, or
+    ``True``/``None`` when acquisition failed without a confirmed holder
+    (``hermes_state.try_acquire_compression_lock`` catches ``sqlite3.Error``
+    internally and returns ``False``, so a failed acquire is NOT proof that
+    another compression is running). The two cases must be worded
+    differently: claiming "already in progress" on an unconfirmed failure
+    misdirects the user when the real problem is a broken lock subsystem.
+    """
+    holder = (
+        lock_signal
+        if isinstance(lock_signal, str) and lock_signal.strip()
+        else None
+    )
+    if holder:
+        return (
+            f"⏳ Compression already in progress for this session "
+            f"(holder: {holder}). Please wait for it to finish."
+        )
+    return (
+        "⏳ Compression skipped: could not acquire this session's "
+        "compression lock. Another compression may still be running, or "
+        "the lock check failed — try again shortly."
+    )
+
+
 def summarize_manual_compression(
     before_messages: Sequence[dict[str, Any]],
     after_messages: Sequence[dict[str, Any]],