fix(honcho): buffer partial memory-context spans across stream deltas

sanitize_context() uses a non-greedy block regex that needs both
<memory-context> open and close tags present in a single string. When a
provider streams the fenced memory block across multiple deltas (typical
for recalled-context leaks — the payload often arrives in 10+ 1-80 char
chunks), the per-delta sanitize stripped the lone open/close tags via
_FENCE_TAG_RE but let the payload in between flow straight to the UI.

Adds StreamingContextScrubber: a small stateful scrubber that tracks
open/close tag pairs across deltas, holds back partial-tag tails at
chunk boundaries, and discards span contents wholesale (including the
system-note line that fragments across deltas).

Wired into _fire_stream_delta; reset per user turn; benign trailing
partial-tag tails are flushed at the end of each model call.  Mid-span
interruption (provider drops closing tag) drops the orphaned content
rather than leaking it — truncated answer > leaked memory.

Follow-up to #13672 (@dontcallmejames).

agent/memory_manager.py

@@ -63,6 +63,117 @@ def sanitize_context(text: str) -> str:
     return text
 
 
+class StreamingContextScrubber:
+    """Stateful scrubber for streaming text that may contain split memory-context spans.
+
+    The one-shot ``sanitize_context`` regex cannot survive chunk boundaries:
+    a ``<memory-context>`` opened in one delta and closed in a later delta
+    leaks its payload to the UI because the non-greedy block regex needs
+    both tags in one string.  This scrubber runs a small state machine
+    across deltas, holding back partial-tag tails and discarding
+    everything inside a span (including the system-note line).
+
+    Usage::
+
+        scrubber = StreamingContextScrubber()
+        for delta in stream:
+            visible = scrubber.feed(delta)
+            if visible:
+                emit(visible)
+        trailing = scrubber.flush()  # at end of stream
+        if trailing:
+            emit(trailing)
+
+    The scrubber is re-entrant per agent instance.  Callers building new
+    top-level responses (new turn) should create a fresh scrubber or call
+    ``reset()``.
+    """
+
+    _OPEN_TAG = "<memory-context>"
+    _CLOSE_TAG = "</memory-context>"
+
+    def __init__(self) -> None:
+        self._in_span: bool = False
+        self._buf: str = ""
+
+    def reset(self) -> None:
+        self._in_span = False
+        self._buf = ""
+
+    def feed(self, text: str) -> str:
+        """Return the visible portion of ``text`` after scrubbing.
+
+        Any trailing fragment that could be the start of an open/close tag
+        is held back in the internal buffer and surfaced on the next
+        ``feed()`` call or discarded/emitted by ``flush()``.
+        """
+        if not text:
+            return ""
+        buf = self._buf + text
+        self._buf = ""
+        out: list[str] = []
+
+        while buf:
+            if self._in_span:
+                idx = buf.lower().find(self._CLOSE_TAG)
+                if idx == -1:
+                    # Hold back a potential partial close tag; drop the rest
+                    held = self._max_partial_suffix(buf, self._CLOSE_TAG)
+                    self._buf = buf[-held:] if held else ""
+                    return "".join(out)
+                # Found close — skip span content + tag, continue
+                buf = buf[idx + len(self._CLOSE_TAG):]
+                self._in_span = False
+            else:
+                idx = buf.lower().find(self._OPEN_TAG)
+                if idx == -1:
+                    # No open tag — hold back a potential partial open tag
+                    held = self._max_partial_suffix(buf, self._OPEN_TAG)
+                    if held:
+                        out.append(buf[:-held])
+                        self._buf = buf[-held:]
+                    else:
+                        out.append(buf)
+                    return "".join(out)
+                # Emit text before the tag, enter span
+                if idx > 0:
+                    out.append(buf[:idx])
+                buf = buf[idx + len(self._OPEN_TAG):]
+                self._in_span = True
+
+        return "".join(out)
+
+    def flush(self) -> str:
+        """Emit any held-back buffer at end-of-stream.
+
+        If we're still inside an unterminated span the remaining content is
+        discarded (safer: leaking partial memory context is worse than a
+        truncated answer).  Otherwise the held-back partial-tag tail is
+        emitted verbatim (it turned out not to be a real tag).
+        """
+        if self._in_span:
+            self._buf = ""
+            self._in_span = False
+            return ""
+        tail = self._buf
+        self._buf = ""
+        return tail
+
+    @staticmethod
+    def _max_partial_suffix(buf: str, tag: str) -> int:
+        """Return the length of the longest buf-suffix that is a tag-prefix.
+
+        Case-insensitive.  Returns 0 if no suffix could start the tag.
+        """
+        tag_lower = tag.lower()
+        buf_lower = buf.lower()
+        max_check = min(len(buf_lower), len(tag_lower) - 1)
+        for i in range(max_check, 0, -1):
+            if tag_lower.startswith(buf_lower[-i:]):
+                return i
+        return 0
+
+
 def build_memory_context_block(raw_context: str) -> str:
     """Wrap prefetched memory in a fenced block with system note.