governance/datasheet: _PROJECT_ROOT path index is off-by-one (parents[4] → parents[3])

[Goldokpa]

Follow-up from review of #52.

In src/climatevision/governance/datasheet.py:

_PROJECT_ROOT = Path(__file__).resolve().parents[4]
_DEFAULT_OUTPUT_DIR = _PROJECT_ROOT / "outputs" / "datasheets"

The file lives at src/climatevision/governance/datasheet.py. Walking parents:

• [0] = governance
• [1] = climatevision
• [2] = src
• [3] = repo root
• [4] = parent of repo root

So _DEFAULT_OUTPUT_DIR lands outside the repo. Should be parents[3].

The existing tests don't catch this because every test passes an explicit tmp_path for output_dir, so the default path is never exercised.

Acceptance criteria:

• Fix the index in datasheet.py.
• Add a unit test that calls write_datasheet(sheet) with no output_dir and asserts the resulting path is inside the repo (e.g. under <repo>/outputs/datasheets/). Use monkeypatch on _DEFAULT_OUTPUT_DIR or assert relative-to-repo-root rather than touching the real outputs/ dir.
• Audit scripts/generate_datasheet.py for the same pattern just in case.

Nice-to-haves (optional, can be split out):

• scripts/generate_datasheet.py defines logger but only uses print — wire one or the other.
• _coerce_config could give a friendlier error if the manifest path doesn't exist.
• Confirm the rendered-markdown output for nested dicts (currently embedded as a JSON code fence) is the intended format.