docs: finish private working-note archive and retain active guidance

.gitignore

@@ -168,3 +168,6 @@ clouds.yaml
 secure.yaml
 **/clouds.yaml
 **/secure.yaml
+
+# Optional local agent context and private research routes
+/AGENTS_Private.md

AGENTS.md

@@ -72,6 +72,12 @@ The controlling design rule is:
 
 ## 2. Grounding and reading
 
+If `AGENTS_Private.md` exists at this repository's root, read it for supplementary
+local context and research routes. It is gitignored and optional; if absent,
+continue with the public guidance. Use relevant accessible research as background
+without treating it as governing authority or copying private material into
+public artefacts.
+
 Every agent must read this file before acting. Then use progressive disclosure:
 
 1. Inspect the current repo/worktree and the live authority or evidence surface

ci/known_test_failures.txt

@@ -24,7 +24,7 @@
 # being red on day one. It is a ratchet in the useful direction: the
 # accompanying test asserts it never grows. Removing an entry is the only
 # permitted edit. A guardrail that certifies its own growth is the failure mode
-# in docs/engineering/reliability_ratchet_articles_and_cases.md, so raising this
+# in the historical Reliability Ratchet case record (Von commit 5b2f8b976069), so raising this
 # baseline is deliberately not an option.
 #
 # Splitting this into "broken" and "needs a database" is worth doing; it would