feat(acl): shared records are search-only and always labelled as someone else's

Narrows the b7d6fc7 widening per the operator's call, and fixes a regression it
introduced. Decision recorded as note 2094.

Two scopes now, deliberately different:

  readable_notes_clause  — everything the ACL permits, including records reached
                           only via a direct/group note share. For EXPLICIT acts:
                           a search the caller typed, a fetch by id.
  browsable_notes_clause — the caller's own records plus anything in a project
                           they can reach. For PASSIVE surfaces: browse lists,
                           facet counts, the process->skill manifest.

The split is a trust boundary. Anything appearing unasked — in your own list,
your own counts, or as a skill installed on your machine — reads as material you
endorsed. A one-off someone shared with you hasn't earned that standing, so it
waits until you go looking. This also dissolves the shared-Process problem by
construction rather than by special case: the manifest is a passive surface, so a
directly-shared Process is never installed as an auto-surfacing skill.

Regression fix (#2093): b7d6fc7 widened the list queries but left the fetch path
owner-only, so on the MCP path a record could be listed and then not opened —
get_snippet raised not-found, get_process couldn't resolve, and the manifest
emitted stubs whose get_process call would fail. snippets.get_snippet and
notes.resolve_process now resolve the read scope. delete_snippet gained an
explicit can_write_note guard, since being able to SEE a shared snippet must not
imply being able to bin it.

Provenance, so nothing arrives looking like the operator's own work:
- access.describe_provenance / label_shared_items add shared/owner/permission;
  labelling costs no query when everything is the caller's own.
- MCP: get_snippet, list_snippets, get_process and list_processes carry it, and
  get_process now says outright NOT to follow a shared process verbatim — its
  follow-as-written contract was the sharpest instance of the problem.
- The skill stub for a shared Process names its author and asks for a go-ahead,
  instead of describing it as "the operator's saved Scribe process".
- Policy stated once in the MCP _INSTRUCTIONS and the reusing-code skill: a
  shared record is that person's suggestion, weigh it, attribute it, ask before
  adopting it.
- UI: shared snippets show "by <owner>" in the list and a notice above the code
  in the detail view, reusing SharedWithMeView's vocabulary.

Plugin 0.1.15 -> 0.1.16 (skill text changed).

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

src/scribe/routes/snippets.py

@@ -19,7 +19,11 @@ from scribe.routes.utils import not_found, parse_pagination
 from scribe.services import dedup as dedup_svc
 from scribe.services import snippets as snippets_svc
 from scribe.services import systems as systems_svc
-from scribe.services.access import can_write_note
+from scribe.services.access import (
+    can_write_note,
+    describe_provenance,
+    label_shared_items,
+)
 from scribe.services.notes import get_note_for_user
 
 logger = logging.getLogger(__name__)
@@ -56,6 +60,9 @@ async def list_snippets_route():
     items, total = await snippets_svc.list_snippets(
         uid, q=q, tag=tag, limit=limit, offset=offset, project_id=project_id,
     )
+    # Mark rows owned by someone else so the UI can show whose they are — an
+    # unmarked row in your own list reads as one you recorded and vetted.
+    items = await label_shared_items(uid, items)
     return jsonify({"snippets": items, "total": total})
 
 
@@ -134,6 +141,7 @@ async def get_snippet_route(snippet_id: int):
         s.to_dict()
         for s in await systems_svc.list_record_systems(note.user_id, snippet_id)
     ]
+    data.update(await describe_provenance(uid, note))
     return jsonify(data)