feat(Agents): structure agents docs with progressive disclosure (#26946)

.agents/docs/README.md

@@ -0,0 +1,20 @@
+# .agents/docs
+
+Task-scoped agent guidance, routed from AGENTS.md's "Mandatory reading per task". Keep every doc
+compact and agent-agnostic.
+
+## Taxonomy
+
+- Root — task-type docs (`build.md`, `cpp-guidelines.md`, `sql-guidelines.md`, …), kebab-case.
+  `<lang>-<topic>.md` is reserved for language-scoped specializations (e.g. `cpp-scripts.md`).
+- `systems/` — cross-language subsystem docs, plain kebab-case subsystem names
+  (e.g. `battleground.md`).
+
+## Placing new guidance
+
+- The most specific applicable doc wins.
+- Generic language lesson → the language doc (e.g. generic C++ → `cpp-guidelines.md`).
+- Subsystem-specific lesson → `systems/<subsystem>.md`; create it if missing.
+- Extend an existing doc before creating a new one.
+- A new doc REQUIRES adding its routing bullet to AGENTS.md's "Mandatory reading per task" in the
+  same change.