Add a "Code review pattern" section that tells the opencode agent to give
claude read-only access to the repo (Read/Grep/Glob plus a narrow git
allowlist) instead of gathering a full `git diff` and piping it in. The
piped-diff form loses surrounding-file context, bloats the prompt, and
falls over on large branches. Also adds a matching entry to "Don'ts" and
caveats the existing pipe-stdin example.
Motivated by an opencode run that collected the entire branch diff as
stdin instead of pointing claude at the working tree.
