fix(compression): notify gateway users when summary generation fails

When auxiliary compression's summary LLM call fails (e.g. model 404,
auxiliary model misconfigured), the compressor still drops the selected
turns and inserts a static fallback placeholder — the dropped context
is unrecoverable.

Previously the only signal of this was a WARNING in agent.log. Gateway
users (Telegram/Discord/etc.) had no way to know context was lost
because the existing _emit_warning path requires a status_callback,
and the gateway hygiene path uses a temporary _hyg_agent with
quiet_mode=True and no callback wired up.

Changes:
- ContextCompressor: track _last_summary_fallback_used and
  _last_summary_dropped_count on each compress() call. Cleared at the
  start of compress() and on session reset.
- gateway/run.py hygiene: after auto-compress, inspect the temp
  agent's compressor; if fallback was used, send a visible ⚠️ warning
  to the user via the platform adapter (TG/Discord/etc.) including
  dropped count and the underlying error.
- gateway/run.py /compress: append the same warning to the manual
  compress reply so users running /compress see the failure too.

Acceptance:
- Summary success: no user-visible warning (unchanged).
- Summary failure on gateway hygiene: user receives a TG/Discord
  message with dropped count + error + remediation hint.
- Summary failure on /compress: warning appended to the command reply.
- CLI status_callback / _emit_warning path is untouched.
- Test coverage: two new tests verify the tracking fields are set on
  failure and cleared on subsequent success.

agent/context_compressor.py

@@ -338,6 +338,8 @@ class ContextCompressor(ContextEngine):
         self._context_probe_persistable = False
         self._previous_summary = None
         self._last_summary_error = None
+        self._last_summary_dropped_count = 0
+        self._last_summary_fallback_used = False
         self._last_compression_savings_pct = 100.0
         self._ineffective_compression_count = 0
 
@@ -441,6 +443,11 @@ class ContextCompressor(ContextEngine):
         self._ineffective_compression_count: int = 0
         self._summary_failure_cooldown_until: float = 0.0
         self._last_summary_error: Optional[str] = None
+        # When summary generation fails and a static fallback is inserted,
+        # record how many turns were unrecoverably dropped so callers
+        # (gateway hygiene, /compress) can surface a visible warning.
+        self._last_summary_dropped_count: int = 0
+        self._last_summary_fallback_used: bool = False
 
     def update_from_response(self, usage: Dict[str, Any]):
         """Update tracked token usage from API response."""
@@ -1196,6 +1203,10 @@ The user has requested that this compaction PRIORITISE preserving all informatio
                 related to this topic and be more aggressive about compressing
                 everything else.  Inspired by Claude Code's ``/compact``.
         """
+        # Reset per-call summary failure state — callers inspect these fields
+        # after compress() returns to decide whether to surface a warning.
+        self._last_summary_dropped_count = 0
+        self._last_summary_fallback_used = False
         n_messages = len(messages)
         # Only need head + 3 tail messages minimum (token budget decides the real tail size)
         _min_for_compress = self.protect_first_n + 3 + 1
@@ -1274,6 +1285,8 @@ The user has requested that this compaction PRIORITISE preserving all informatio
             if not self.quiet_mode:
                 logger.warning("Summary generation failed — inserting static fallback context marker")
             n_dropped = compress_end - compress_start
+            self._last_summary_dropped_count = n_dropped
+            self._last_summary_fallback_used = True
             summary = (
                 f"{SUMMARY_PREFIX}\n"
                 f"Summary generation was unavailable. {n_dropped} conversation turns were "