Add testing and type annotation conventions to AGENTS.md #8

What happened

The code agent created PR #7 implementing personalized greetings (issue #4). The first review found two low-severity findings that the code agent could have avoided:

1. Test naming inconsistency (tests/test_main.py): tests for get_greeting_name() used the pattern test_<function>_<behavior> while tests for main() omitted the function name prefix. The review requested consistent naming.
2. Missing return type annotation (src/fullsend_python/main.py): get_greeting_name() had -> str but main() lacked -> None, creating inconsistent annotation coverage.

Both findings were addressed by the fix agent (cost $0.76) and verified by the re-review (cost $2.92). The repo's AGENTS.md currently documents project structure and build commands but contains no coding conventions that would guide the code agent on test naming or type annotations.

What could go better

The code agent has no repo-specific conventions to follow for test naming patterns or type annotation requirements. Without explicit guidance in AGENTS.md, the agent made reasonable but inconsistent choices — using two different test naming patterns in the same file and annotating one function but not another.

These are minor findings individually, but they contributed to the review-fix-re-review cycle that cost $3.68. If AGENTS.md documented the expected conventions, the code agent would likely follow them on the first attempt, avoiding the rework.

Confidence: moderate. The code agent generally follows AGENTS.md guidance well (it correctly used uv, placed code under src/fullsend_python/, etc.). Adding conventions there is the natural mechanism for this repo. However, this is a young repo with only 2 closed issues, so conventions may evolve — overly prescriptive guidance now could become stale quickly.

Proposed change

Add a Conventions subsection to AGENTS.md covering testing and type annotations. Specifically:

## Conventions

- Use `uv` for all dependency and lifecycle management.
- Source code lives under `src/fullsend_python/`.
- Keep the application entry point in `src/fullsend_python/main.py`.
- Document requirements in the `docs/` folder.
- All functions must have return type annotations (use `-> None` for functions that return nothing).
- Test functions follow the pattern `test_<function_name>_<behavior>` (e.g., `test_get_greeting_name_returns_username`).
- Run tests with `uv run pytest tests/ -v`.

This adds three new bullet points to the existing Conventions section: return type annotations, test naming pattern, and the test run command. The test command is also useful context for the code agent when adding tests.

Validation criteria

On the next code agent run that creates or modifies Python functions and tests in this repo, verify:

1. All new functions include return type annotations (including -> None).
2. All new test functions follow the test_<function_name>_<behavior> naming pattern.
3. The review agent does not flag naming-convention or api-shape findings for the same issues.

Sample size: the next 3 code agent PRs that touch Python source and test files.

Generated by retro agent from #7