CI: document PR title convention for CI-only PRs

CLAUDE.md

@@ -13,3 +13,28 @@ failures before treating the work as done:
 
 Do not consider a push complete until its CI run is green (or the failure is
 understood and explicitly accepted by the user).
+
+## PR title convention for CI-only PRs
+
+A PR gets a title starting with `CI: ` (capital, colon, space) when its diff
+doesn't touch anything involved with the actual code itself — this is a HARD
+boundary, not a synonym for "lives under `.github/workflows/`":
+
+- If a change touches ANYTHING that's part of the actual code — SQL source,
+  `object_reference.control`, anything that affects what gets installed or
+  how it behaves at runtime — it is NOT CI-only, full stop. When unsure,
+  **always err on the side of NOT CI-only.**
+- Files elsewhere that genuinely don't touch the code qualify too, not just
+  `.github/workflows/*`: e.g. `.gitignore`, this `CLAUDE.md`, other pure
+  documentation/metadata.
+- **`test/` is treated as NOT CI-only, even though it's a bit of a grey
+  area.** Test files aren't the shipped code itself, but default to
+  excluding them from the prefix rather than trying to judge case by case.
+- A PR that's CI-*motivated* but also touches a real code/test file (a
+  `bin/` script a workflow calls, a linter's `Makefile` wiring if it affects
+  what ships, a submodule) is NOT CI-only under this reading, even though CI
+  is the reason it exists — don't stretch the prefix to cover those.
+
+Check the actual file list
+(`gh pr view <n> --json files --jq '.files[].path'`) before applying it, don't
+guess from the title/description alone.