docs: add commit message quality rules to git instructions

- read diff before writing, only include what's in the diff
- no CHANGES: headers, no speculative outcome sections
- bullet points state what changed, not why or how

.ai/instructions/skills/git.instructions.md

@@ -56,3 +56,23 @@ Format:
 ```
 
 Types: `feat`, `fix`, `docs`, `refactor`, `chore`
+
+### Commit message rules
+
+**Only include what actually changed in this commit.**
+
+- Run `git diff --staged` (or `git diff` if nothing staged) and read it fully before writing
+- List only files and lines that appear in the diff
+- If a file appears in the diff but only has whitespace or comment changes, omit it from the body
+- Never mention changes that are not in the diff — even if you remember discussing them earlier in the session
+- If unsure whether something changed, check the diff — do not guess
+
+**Format rules:**
+- Subject line: max ~72 chars, imperative mood ("Fix", "Add", "Remove" — not "Fixed", "Added")
+- Body: optional, only when the *why* is not obvious from the diff
+- No bullet-pointed list of every file changed unless the diff spans many unrelated files
+- No "CHANGES:" headers, no file-by-file breakdowns for single-topic commits
+- No "Expected:", "Result:", "Impact:" or any speculative outcome sections — commit messages describe what changed, not what is predicted to happen
+- Bullet points state *what* changed, not *why it was wrong* or *how it works* — keep each line short and factual
+  - ✅ `use MC_HOST_minio for S3 auth`
+  - ❌ `Use MC_HOST_minio env var for direct S3 auth (no mc alias set)`