docs(adr): record that Research doc names one Research registry

ADR-0028 records the grammar decided for #121 and the alternatives
rejected at each step: what `Research doc:` refers to, how an entry with
no registry declares that (`none` plus `Basis:`), the FAIL and INFO
tiers, the corpus-wide sweep gate, parser parity, retiring check 8, the
`(removed in <sha>)` escape for Basis paths, and removing the lint entry
that had no verifiable basis.

Add the Research registry term to CONTEXT.md, since "registry" had no
definition and "research doc" was being used for both the registry and
the topic docs it digests.

Refs: #121
ADR: 0028
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EGHFJextYtVQseaHPDDhxB

CONTEXT.md

@@ -81,6 +81,13 @@ topic docs and a `sources.md`; the author skill records which sources informed w
 and internally consistent.
 _Avoid_: sources, citations, attribution
 
+**Research registry**:
+A plugin's research `sources.md` (e.g. `plugins/git/docs/research/docs/git/sources.md`), whose `## H2`
+headings are the source slugs. A skill's `Research doc:` field names exactly one, and
+`factory-audit` resolves each entry's slug against it. An entry with no registry declares
+`Research doc: none` and names what it was actually drawn from in `Basis:`.
+_Avoid_: research doc, sources file, topic doc (a topic doc is a digest of sources, not the registry)
+
 ### Governance
 
 **HITL** (human-in-the-loop):