docs(agents): drop inline ADR citations from AGENTS.md

The remaining docs/adr/ references were citations inside architecture prose,
or an index of them. The prose stays; the pointers go.

Author: lesnik512

AGENTS.md

@@ -82,7 +82,7 @@ review time.
   `ty`, which silently accepts the latter without checking the rule
 - Docstrings: public API documents the contract; internal helpers get a one-line contract, plus at
   most 1–2 lines for a genuinely non-obvious constraint. Never narrate implementation or justify
-  code to a reviewer — cross-file rationale lives in an `INVARIANT:` test docstring or an ADR
+  code to a reviewer — cross-file rationale lives in an `INVARIANT:` test docstring
 
 ## Agent skills