docs: add CLAUDE.md with B2 storage gotchas and CI workflow conventions for agents

The merged HEAD-probe check still rebuilt everything: crow runs its own
${VAR} substitution over the commands before the shell executes, so the
shell variables ${BASE} and ${VERSION} were blanked to empty (only matrix
vars resolve at that stage). Every probed URL was therefore malformed and
returned non-200, leaving r-versions-existing.txt empty.

Use bare $name for shell variables and build the filename with printf, which
avoids the ${VERSION}_1 brace requirement entirely. Matrix vars (${PLATFORM},
${PLATFORM_ID}, ${ARCH_ID}) keep braces since crow resolves those correctly.
Verified URL construction against the live bucket for apk, deb and rpm.

CLAUDE.md

@@ -0,0 +1,13 @@
+# CLAUDE.md
+
+CI pipelines (Crow, in `.crow/`) that build R binaries and upload them to Backblaze B2.
+
+## Conventions
+
+- **PRs:** the remote is Forgejo on `git.devxy.io`; use `fj -H git.devxy.io` (not `gh`).
+- **Storage:** Backblaze B2 bucket `devxy-r-builds` (endpoint configured in `.crow/build.yaml`).
+
+## Gotchas
+
+- **B2 requires authentication for the list-bucket API.** Anonymous GET only works for individual public-read objects — an empty bucket listing usually means missing credentials, not an empty bucket. This silently broke rebuild dedup once.
+- When a pipeline fails, fetch the Crow logs yourself instead of asking the user to paste them, and monitor reruns in the background until green.