fix: insert static fallback when compression summary fails

When _generate_summary() failed (no provider, timeout, model error),
the compressor silently dropped all middle turns with just a debug
log. The agent would then see head + tail with no explanation of the
gap, causing total context amnesia (generic greetings instead of
continuing the conversation).

Now generates a static fallback marker that tells the model context
was lost and to continue from the recent tail messages. The fallback
flows through the same role-alternation logic as a real summary so
message structure stays valid.

agent/context_compressor.py

@@ -691,33 +691,43 @@ Write only the summary body. Do not include any preamble or prefix."""
                 )
             compressed.append(msg)
 
-        _merge_summary_into_tail = False
-        if summary:
-            last_head_role = messages[compress_start - 1].get("role", "user") if compress_start > 0 else "user"
-            first_tail_role = messages[compress_end].get("role", "user") if compress_end < n_messages else "user"
-            # Pick a role that avoids consecutive same-role with both neighbors.
-            # Priority: avoid colliding with head (already committed), then tail.
-            if last_head_role in ("assistant", "tool"):
-                summary_role = "user"
-            else:
-                summary_role = "assistant"
-            # If the chosen role collides with the tail AND flipping wouldn't
-            # collide with the head, flip it.
-            if summary_role == first_tail_role:
-                flipped = "assistant" if summary_role == "user" else "user"
-                if flipped != last_head_role:
-                    summary_role = flipped
-                else:
-                    # Both roles would create consecutive same-role messages
-                    # (e.g. head=assistant, tail=user — neither role works).
-                    # Merge the summary into the first tail message instead
-                    # of inserting a standalone message that breaks alternation.
-                    _merge_summary_into_tail = True
-            if not _merge_summary_into_tail:
-                compressed.append({"role": summary_role, "content": summary})
-        else:
+        # If LLM summary failed, insert a static fallback so the model
+        # knows context was lost rather than silently dropping everything.
+        if not summary:
             if not self.quiet_mode:
-                logger.debug("No summary model available — middle turns dropped without summary")
+                logger.warning("Summary generation failed — inserting static fallback context marker")
+            n_dropped = compress_end - compress_start
+            summary = (
+                f"{SUMMARY_PREFIX}\n"
+                f"Summary generation was unavailable. {n_dropped} conversation turns were "
+                f"removed to free context space but could not be summarized. The removed "
+                f"turns contained earlier work in this session. Continue based on the "
+                f"recent messages below and the current state of any files or resources."
+            )
+
+        _merge_summary_into_tail = False
+        last_head_role = messages[compress_start - 1].get("role", "user") if compress_start > 0 else "user"
+        first_tail_role = messages[compress_end].get("role", "user") if compress_end < n_messages else "user"
+        # Pick a role that avoids consecutive same-role with both neighbors.
+        # Priority: avoid colliding with head (already committed), then tail.
+        if last_head_role in ("assistant", "tool"):
+            summary_role = "user"
+        else:
+            summary_role = "assistant"
+        # If the chosen role collides with the tail AND flipping wouldn't
+        # collide with the head, flip it.
+        if summary_role == first_tail_role:
+            flipped = "assistant" if summary_role == "user" else "user"
+            if flipped != last_head_role:
+                summary_role = flipped
+            else:
+                # Both roles would create consecutive same-role messages
+                # (e.g. head=assistant, tail=user — neither role works).
+                # Merge the summary into the first tail message instead
+                # of inserting a standalone message that breaks alternation.
+                _merge_summary_into_tail = True
+        if not _merge_summary_into_tail:
+            compressed.append({"role": summary_role, "content": summary})
 
         for i in range(compress_end, n_messages):
             msg = messages[i].copy()