docs: add developer testing guidelines (#70)

Add docs/developer-testing-guidelines.md covering test commands, targeted
runs, syntax/diff checks, adding MCP tool tests, safe API/auth mocking,
profile/allowed-operation gate tests, self-review/self-merge gate tests,
no-secret regression expectations, unit vs future Docker integration tests,
and read-only vs mutating tool expectations. Link it from the README Tests
section and note the suite table is non-exhaustive.

Documentation only; no code behavior changed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -381,8 +381,15 @@ python3 -m pytest tests/ -v
 | `test_python_cli.py`    | `close_issue.py` + `mark_issue.py` CLI validation       |
 | `test_mirror_refs.py`   | Flags, safety defaults, local integration tests |
 
+(Core suites — the table is non-exhaustive; see `tests/` for the full set.)
+
 All tests mock network and keychain access — no real API calls are made.
 
+For how to write tests — mocking the API/auth safely, testing profile and
+self-review/self-merge gates, no-secret regression expectations, and unit vs.
+integration guidance — see
+[`docs/developer-testing-guidelines.md`](docs/developer-testing-guidelines.md).
+
 ## Troubleshooting
 
 ### macOS: `com.apple.provenance` blocks Python execution (#3)