fix(config): use [metadata] section header in gittuf and hello TOMLs

[Marc-cn]

Summary

Both plugin TOMLs used [framework] where FrameworkConfig requires [metadata], so both failed schema validation once the packaging fix made them loadable.

• packages/darnit-gittuf/src/darnit_gittuf/gittuf.toml:1
• packages/darnit-hello/src/darnit_hello/hello.toml:16

Two lines, header rename only.

Fixes #361

Type of Change

• Bug fix (non-breaking change fixing an issue)
• New feature (non-breaking change adding functionality)
• Breaking change (fix or feature causing existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

Framework Changes Checklist

If this PR modifies the darnit framework (packages/darnit/):

• n/a — no changes under packages/darnit/, only two plugin TOMLs

Control/TOML Changes Checklist

If this PR modifies controls or TOML configuration:

• Control metadata defined in TOML (not Python code) — unchanged, headers only
• SARIF fields (description, severity, help_url) included where appropriate — n/a, no control definitions touched
• Ran validation to confirm TOML schema compliance

Testing

• Tests pass locally
• Added tests for new functionality (if applicable) — n/a
• Linting passes (uv run ruff check .)

- gittuf
  Display: Gittuf Policy Checks
  Version: 0.1.0
  Spec: gittuf v0.1
  Controls: 3
- hello
  Display: Hello — Minimal Darnit Plugin
  Version: 0.1.0
  Spec: hello v0.1
  Controls: 1

Also confirmed directly:

>>> load_framework_by_name('gittuf').metadata
name='gittuf' display_name='Gittuf Policy Checks' version='0.1.0'
schema_version='0.1.0-alpha' spec_version='gittuf v0.1' ... url=None

uv run pytest tests/darnit_gittuf -q passes. There is no tests/darnit_hello directory.

Additional Notes

[mlieberman85]

Confirmed the fix locally:

• After checkout, load_framework_by_name('gittuf').metadata and load_framework_by_name('hello').metadata both return populated FrameworkMetadata with the expected name/display_name/version/spec_version.
• pytest tests/darnit_gittuf -q -- 21 pass.
• No stragglers: grep -rn '^\[framework\]' packages/ scripts/ docs/ returns nothing after the change; baseline and reproducibility already use [metadata], so this brings gittuf and hello in line with the schema (FrameworkConfig.metadata: FrameworkMetadata, framework_schema.py:1477).
• CI "Plugin discovery smoke (darnit-hello)" check is the equivalent of a hello test-suite here and it's green.

Two lines, right fix, closes #361 cleanly. LGTM.