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.

cli.py

@@ -10180,17 +10180,26 @@ class HermesCLI(CLIAgentSetupMixin, CLICommandsMixin, CLIBillingMixin):
                 # If _compress_context returned unchanged because a
                 # concurrent compression lock is held, tell the user
                 # clearly instead of showing the misleading
-                # "No changes from compression" no-op text.
+                # "No changes from compression" no-op text. The wording
+                # distinguishes a confirmed holder from an unconfirmed
+                # acquisition failure (describe_compression_lock_skip).
                 if getattr(self.agent, "_compression_skipped_due_to_lock", None):
-                    holder = self.agent._compression_skipped_due_to_lock
-                    if isinstance(holder, str):
-                        print(f"  ⏳ Compression already in progress for this "
-                              f"session (holder: {holder}). "
-                              f"Please wait for it to finish.")
-                    else:
-                        print(f"  ⏳ Compression already in progress for this "
-                              f"session. Please wait for it to finish.")
+                    from agent.manual_compression_feedback import (
+                        describe_compression_lock_skip,
+                    )
+                    print(
+                        "  "
+                        + describe_compression_lock_skip(
+                            self.agent._compression_skipped_due_to_lock
+                        )
+                    )
                     self.agent._compression_skipped_due_to_lock = None
+                    # No boundary was committed on a lock-skip; discard the
+                    # deferred context-engine notification (exactly-once).
+                    finalize_context_engine_compression_notification(
+                        self.agent,
+                        committed=False,
+                    )
                     return
 
                 if partial and tail: