feat: add code-review-checklist example skill for code review guidance - #903

Merging? Review??

Thanks for the contribution. A few concerns:

• The PR title says "add code-review-checklist example skill" but the diff adds three separate skills (api-design-review, code-review-checklist, testing-strategies). Please scope the PR to what the title describes, or explain the broader intent in the PR body (which is currently empty).
• These are pure documentation/prompt-template additions with no functional code, so there's nothing to test, but it's still worth confirming these follow the exact SKILL.md schema/conventions used by any existing skills in common/src/templates/initial-agents-dir/skills/ (if any precedent exists) - the diff doesn't show whether this directory previously had a skill index, registration step, or naming convention that needs to be respected. If a skill needs to be registered somewhere else (a manifest, README listing, etc.), that's missing here.
• Content quality is fine and generic (standard REST/testing/code-review advice), but it isn't obviously specific to Codebuff's actual workflows or tools beyond the generic read_files/code_search/run_terminal_command mentions repeated near-verbatim across all three files - worth checking whether this is genuinely useful guidance for the agent or filler.
• Since this is a squash-mirror, a maintainer would need to manually decide whether to port one, some, or all three skills - bundling unrelated skills under a mismatched title makes that harder to reason about and review.

Suggest splitting into one PR per skill (or renaming this PR to reflect all three), and clarifying in the body why these specific three skills were chosen and how they fit the existing skills directory conventions.