fix(aura-research): process.ref hint states the tagged { content_id } shape

campaign introspect (--block std::process_ref and --unwired) described
the slot as "content id of a process document" — a plausible bare-string
reading the validator rejects; the required {"content_id":"..."} tagging
appeared in no introspection output (binary-only M1 fieldtest finding).

The SlotKind::ContentRef label and the open-slot hint now lead with the
tagged { content_id } shape. Deliberately narrower than the issue's
suggestion of mirroring std::strategy's { content_id } | { identity_id }
form: validate_campaign refuses an identity_id process ref
(DocFault::ProcessRefMustBeContentId) and a pinned test forbids
advertising it, so the hint states the one accepted key only (C29
honesty; correction minuted on the issue). RED-first: the extended
process_ref_describe_advertises_content_id_only assertion failed against
the old label before the fix.

closes #329

crates/aura-cli/tests/research_docs.rs

@@ -996,7 +996,7 @@ fn campaign_introspect_unwired_reports_the_spec_example_slots() {
     write_doc(&dir, "draft.campaign.json", draft);
     let (out, code) = run_code_in(&dir, &["campaign", "introspect", "--unwired", "draft.campaign.json"]);
     assert_eq!(code, Some(0));
-    assert!(out.contains("open slot: process.ref (required, content id of a process document)"));
+    assert!(out.contains("open slot: process.ref (required, { content_id }: content id of a process document)"));
     assert!(out.contains("open slot: strategies[0].axes.slow (axis declared empty — a P1 axis is a non-empty finite set)"));
 }
 
@@ -1022,7 +1022,7 @@ fn campaign_introspect_unwired_reports_placeholder_refs() {
         "strategy `ref: null` placeholder not enumerated: {out}"
     );
     assert!(
-        out.contains("open slot: process.ref (required, content id of a process document)"),
+        out.contains("open slot: process.ref (required, { content_id }: content id of a process document)"),
         "process `ref: {{}}` placeholder not enumerated: {out}"
     );
 }