fix(acp): an overlay HERMES_HOME stages prompt images in the owner's cache

An overlay task home attaches to the profile its symlinked lock names, but
image staging still used the overlay's ambient HERMES_HOME. The overlay does
not share cache/images, and the owner admits attachments only from its own
image cache, so every image prompt failed with invalid_params. Staging now
runs under the resolved owner home (context-local home override); the owner's
path check is unchanged.

Review finding on #140190 (Greptile)
Refs #134889 (ACP-18)
Refs #140138

acp_adapter/gateway_media.py

@@ -39,16 +39,26 @@ def _take(path):
             os.unlink(path)
 
 
-async def stage_user_content(content):
+async def stage_user_content(content, home=None):
     """Shared-converter output -> ``(text, attachments)`` for ``prompt.submit``.
 
     Text-only prompts stay a plain string. Media parts (image ``data:`` URLs from
     image blocks, image resource links and embedded blobs, and remote image URLs once
     fetched) are staged as bytes in the profile image cache; the authority commits them
     at admission. A remote image URL that cannot be fetched is kept as text so the model
-    still sees it. Only a remote image awaits (its download); every other part stages inline."""
+    still sees it. Only a remote image awaits (its download); every other part stages inline.
+
+    ``home`` is the owner's profile home: a per-task overlay ``HERMES_HOME`` does not share
+    ``cache/images``, and admission accepts only the owner's cache (never relaxed here)."""
     if isinstance(content, str):
         return content, []
+    if home is not None:
+        from hermes_constants import reset_hermes_home_override, set_hermes_home_override
+        token = set_hermes_home_override(home)
+        try:
+            return await stage_user_content(content)
+        finally:
+            reset_hermes_home_override(token)
     import base64
     import binascii
     from gateway.platforms.base import cache_image_from_bytes

acp_adapter/gateway_server.py

@@ -632,7 +632,7 @@ class GatewayACPAgent(GatewayReplyMixin, acp.Agent):
         # submit itself, and the prompt then ends without being sent.
         self._submitting.add(session_id)
         try:
-            staged = await stage_user_content(content)
+            staged = await stage_user_content(content, self._home)
         finally:
             self._submitting.discard(session_id)
         if session_id in self._pending_cancels:

tests/acp_adapter/test_gateway_overlay_home.py

@@ -39,3 +39,27 @@ async def test_overlay_home_attaches_to_the_owner_its_lock_names(daemon, tmp_pat
         saved = await ws.rpc('session.resume', session_id=sid)
         assert 'WS_SHARED overlay' in json.dumps(saved['messages'])
     assert not (overlay[0] / 'gateway.sock').exists() and not (overlay[0] / 'gateway.pid').exists()
+
+
+_ONE_PX_PNG = bytes.fromhex(
+    '89504e470d0a1a0a0000000d49484452000000010000000108060000001f15c4'
+    '890000000a49444154789c6300010000000500010d0a2db40000000049454e44ae426082')
+
+
+@pytest.mark.platforms('linux')
+@pytest.mark.asyncio
+async def test_overlay_home_image_prompt_is_staged_where_the_owner_admits_it(daemon, tmp_path, model_peer):
+    """The overlay does not share ``cache/images``: an image is staged in the owner's cache, the only
+    place its admission accepts (#140190 review)."""
+    import base64
+    overlay = _overlay(daemon, tmp_path)
+    async with editor(overlay, tmp_path) as acp:
+        await acp.rpc('initialize', protocolVersion=1, clientCapabilities={})
+        sid = (await acp.rpc('session/new', cwd=str(tmp_path), mcpServers=[]))['result']['sessionId']
+        turn = await acp.rpc('session/prompt', sessionId=sid, prompt=[
+            {'type': 'text', 'text': 'WS_SHARED overlay image'},
+            {'type': 'image', 'data': base64.b64encode(_ONE_PX_PNG).decode(), 'mimeType': 'image/png'}])
+        assert 'result' in turn, (turn, (tmp_path / 'acp.log').read_text()[-2000:])
+        assert turn['result']['stopReason'] == 'end_turn', turn
+    assert any(daemon[0].glob('cache/images/img_*.png')), list(daemon[0].rglob('*.png'))
+    assert not any(overlay[0].glob('cache/images/*')), list(overlay[0].rglob('*.png'))