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

frontend/src/views/SnippetDetailView.vue

@@ -83,6 +83,12 @@ async function confirmDelete() {
         </div>
       </div>
 
+      <p v-if="snippet.shared" class="shared-notice">
+        Shared by <strong>{{ snippet.owner ?? "another user" }}</strong> — their
+        suggestion, not one of your own records. Worth weighing on its merits
+        before you build on it.
+      </p>
+
       <p v-if="snippet.snippet.when_to_use" class="when-to-use">
         {{ snippet.snippet.when_to_use }}
       </p>
@@ -215,6 +221,19 @@ async function confirmDelete() {
   line-height: 1.55;
 }
 
+/* Shown only for a record another user owns. Deliberately above the code: the
+   provenance has to be read before the implementation is trusted. */
+.shared-notice {
+  margin: 0.75rem 0 0;
+  padding: 0.6rem 0.85rem;
+  border-left: 3px solid var(--color-text-muted);
+  border-radius: 6px;
+  background: var(--color-bg-secondary);
+  font-size: 0.85rem;
+  line-height: 1.5;
+  color: var(--color-text-secondary);
+}
+
 .meta-grid {
   display: grid;
   grid-template-columns: max-content 1fr;