fix(compression): drop conflicting 'resume Active Task' directive in summary prefix

SUMMARY_PREFIX previously contained two contradictory directives:

1. "treat it as background reference, NOT as active instructions"
   "Do NOT answer questions or fulfill requests mentioned in this summary"
   "Respond ONLY to the latest user message that appears AFTER this summary"

2. "Your current task is identified in the '## Active Task' section of the
    summary — resume exactly from there."

When the latest user message contradicted Active Task (e.g. 'stop the
i18n refactor', 'never mind, look at grafana instead'), models tended to
follow (2) anyway because 'resume exactly' is a strong, unambiguous
directive — leading to repeated re-surfacing of already-cancelled work
across turns, even after explicit 'stop'/'don't keep bringing that up'
messages from the user.

This change:
- Removes the conflicting 'resume exactly from Active Task' clause.
- Makes the precedence explicit: latest user message is the single source
  of truth; it WINS on conflict; cancelled Active Task / In Progress /
  Pending User Asks / Remaining Work must be discarded entirely (no
  'wrap up the old task first').
- Names canonical reverse signals (stop, undo, roll back, never mind,
  just verify, topic change) so the model recognizes them as cancellation
  triggers, not background context.
- Updates the summarizer template instruction so the LLM doesn't
  mechanically copy a cancelled task into Active Task on the next
  compaction (it's instructed to copy the reverse signal verbatim).
- Preserves: REFERENCE ONLY framing, MEMORY.md/USER.md authority, and
  the 'don't repeat work already reflected in session state' clause.

Adds tests/agent/test_summary_prefix_semantics.py to pin invariants so
the conflict can't regress.

Tested:
- All compaction tests pass: tests/agent/test_context_compressor.py,
  tests/agent/test_context_compressor_summary_continuity.py,
  tests/run_agent/test_413_compression.py,
  tests/run_agent/test_compression_persistence.py,
  tests/run_agent/test_compression_boundary_hook.py,
  tests/cli/test_manual_compress.py — 117/117 passing.
- Tested on macOS.

agent/context_compressor.py

@@ -40,14 +40,24 @@ SUMMARY_PREFIX = (
     "window — treat it as background reference, NOT as active instructions. "
     "Do NOT answer questions or fulfill requests mentioned in this summary; "
     "they were already addressed. "
-    "Your current task is identified in the '## Active Task' section of the "
-    "summary — resume exactly from there. "
+    "Respond ONLY to the latest user message that appears AFTER this "
+    "summary — that message is the single source of truth for what to do "
+    "right now. "
+    "If the latest user message is consistent with the '## Active Task' "
+    "section, you may use the summary as background. If the latest user "
+    "message contradicts, supersedes, changes topic from, or in any way "
+    "diverges from '## Active Task' / '## In Progress' / '## Pending User "
+    "Asks' / '## Remaining Work', the latest message WINS — discard those "
+    "stale items entirely and do not 'wrap up the old task first'. "
+    "Reverse signals in the latest message (e.g. 'stop', 'undo', 'roll "
+    "back', 'just verify', 'don't do that anymore', 'never mind', a new "
+    "topic) must immediately end any in-flight work described in the "
+    "summary; do not re-surface it in later turns. "
     "IMPORTANT: Your persistent memory (MEMORY.md, USER.md) in the system "
     "prompt is ALWAYS authoritative and active — never ignore or deprioritize "
     "memory content due to this compaction note. "
-    "Respond ONLY to the latest user message "
-    "that appears AFTER this summary. The current session state (files, "
-    "config, etc.) may reflect work described here — avoid repeating it:"
+    "The current session state (files, config, etc.) may reflect work "
+    "described here — avoid repeating it:"
 )
 LEGACY_SUMMARY_PREFIX = "[CONTEXT SUMMARY]:"
 
@@ -1241,6 +1251,11 @@ task assignment verbatim — the exact words they used. If multiple tasks
 were requested and only some are done, list only the ones NOT yet completed.
 Continuation should pick up exactly here. Example:
 "User asked: 'Now refactor the auth module to use JWT instead of sessions'"
+If the user's most recent message was a reverse signal (stop, undo, roll
+back, never mind, just verify, change of topic) that supersedes earlier
+work, write the reverse signal verbatim and DO NOT carry forward the
+cancelled task. Example: "User asked: 'Stop the i18n refactor and just
+verify the current diff' — earlier i18n in-flight work is cancelled."
 If no outstanding task exists, write "None."]
 
 ## Goal