fix(acl): align MCP writes with the share model — editor edits, viewer doesn't

Option B, per the operator. Closes the last inconsistency from the ACL work.

The agent path had drifted into an indefensible position: delete_snippet honoured
editor shares (I made it share-aware so widening the read wouldn't let a VIEWER
trash things) while update_snippet still resolved through the owner-only
notes.get_note. So through an agent you could destroy a colleague's snippet but
not improve it — and the refusal claimed "not found" for a record you could
plainly open.

Now update_snippet, merge_snippets and update_process all resolve the read scope
and then require can_write_note, matching the REST routes and the sharing UI's
own promise that viewer / editor / admin are distinct grants. A viewer grant is
refused with the actual reason ("shared with you read-only — ask its owner for
edit access, or record your own version"), because not-found would send an agent
hunting for a missing id instead of recording its own copy.

Authorised writes are performed as the OWNER, since the underlying note update is
owner-scoped and a shared editor's own id would match nothing.

Merge additionally requires each source to share the TARGET'S owner and to be
writable by the caller — merging trashes the source, so read access isn't enough,
and cross-owner merge stays out of scope (#231). Sources failing either test are
skipped rather than half-merged.

A record the caller cannot read at all still returns not-found rather than
forbidden, so the error can't be used to confirm that an id exists.

Also fixed _fake_snippet's missing user_id proactively — the same
auto-MagicMock-reads-as-foreign trap that broke CI twice (see note 2109).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLwAaV4DQEmVyn496HnEvt

src/scribe/mcp/tools/processes.py

@@ -88,11 +88,21 @@ async def get_process(name_or_id: str) -> dict:
 async def update_process(process_id: int, title: str = "", body: str = "",
                          tags: list[str] | None = None) -> dict:
     """Update a stored process. Only provided fields change — empty title/body
-    leave that field unchanged; pass tags to replace the tag set."""
+    leave that field unchanged; pass tags to replace the tag set.
+
+    Editing another user's process requires an editor or admin share from them; a
+    read-only share is not enough and says so rather than claiming not-found.
+    """
     uid = current_user_id()
-    note = await notes_svc.get_note(uid, process_id)
-    if note is None or note.note_type != "process":
+    loaded = await notes_svc.get_note_for_user(uid, process_id)
+    note = loaded[0] if loaded else None
+    if note is None or note.note_type != "process" or note.deleted_at is not None:
         raise ValueError(f"process {process_id} not found")
+    if not await access_svc.can_write_note(uid, process_id):
+        raise ValueError(
+            f"process {process_id} is shared with you read-only — ask its owner "
+            f"for edit access, or save your own copy with create_process"
+        )
     fields: dict = {}
     if title.strip():
         fields["title"] = title.strip()
@@ -100,8 +110,13 @@ async def update_process(process_id: int, title: str = "", body: str = "",
         fields["body"] = body
     if tags is not None:
         fields["tags"] = tags
-    updated = await notes_svc.update_note(uid, process_id, **fields)
-    return updated.to_dict()
+    # As the owner — update_note is owner-scoped and the write is authorised above.
+    updated = await notes_svc.update_note(note.user_id, process_id, **fields)
+    if updated is None:
+        raise ValueError(f"process {process_id} not found")
+    out = updated.to_dict()
+    out.update(await access_svc.describe_provenance(uid, updated))
+    return out
 
 
 def register(mcp) -> None: