refactor: rename darnit-reproducibility to darnit-amber

[Marc-cn]

The reproducibility module is becoming Amber. This renames it in place - it stays a darnit plugin under packages/, same as the others.

Names chosen to match existing conventions: package darnit-amber (every other plugin is darnit-*), framework amber (every other framework is the bare domain - openssf-baseline, community-spec, gittuf).

What changed

• packages/darnit-reproducibility → packages/darnit-amber, module darnit_reproducibility → darnit_amber, class ReproducibilityImplementation → AmberImplementation
• reproducibility.toml → amber.toml; framework and MCP server name reproducibility → amber
• Entry points under darnit.frameworks and darnit.implementations, plus the implementation's name property - that last one is what discovery.py keys the registry by, so the rename doesn't work without it
• docs/design/reproducibility-attestation-system.md → amber-attestation-system.md
• Display name is now "Amber - Scientific Reproducibility Checks"

Breaking

--framework reproducibility becomes --framework amber. darnit-reproducibility on PyPI is superseded by darnit-amber (neither name exists on TestPyPI yet; see #519).

Upgrading in place needs uv sync --reinstall, not plain uv sync. The old editable install leaves a darnit_reproducibility entry point behind, which then fails to load and masks the new one. Clean environments (CI) aren't affected.

Type of Change

• Refactoring (no functional changes)
• Breaking change (framework name and package name)

Testing

• Tests pass locally
• Linting passes

tests/darnit_amber + tests/darnit/sieve + tests/darnit/test_plugin_handler_registration.py + tests/packaging: 293 passed, 4 skipped. darnit list shows amber with its 5 controls; darnit audit --framework amber runs them end to end.

AI assistance

• No AI assistance was used
• AI assistance was used

Left alone deliberately

• specs/ - 20 files mention the old name, but they're records of what was built at the time. Rewriting them would double the diff
• The audit_reproducibility MCP tool in amber.toml - renaming a tool breaks any agent config that references it. Separate change if we want it.
• Prose about reproducibility as a concept - the plugin still checks reproducibility; only its name changed

[mlieberman85]

This now conflicts with 0.1.1 and 044: reproducibility.toml gained effects/ceiling declarations, and its tests reference the framework by name. Before merge:

1. packaging/pypi/public-packages.txt still lists darnit-reproducibility, so the release preflight and build fail.
2. Upgrade path: pip install -U darnit-mcp would leave darnit-reproducibility installed and failing to load. It needs a final darnit-reproducibility release that depends on darnit-amber, or the distribution name kept.
3. Update the tests that still expect the reproducibility framework name (test_plan_contract.py and the 044 reproducibility tests), and docs/getting-started/using-skills.md.
4. Add a CHANGELOG entry (breaking) and a linked issue or decision record.
5. Rebase after #519 lands; its Dockerfile loop and smoke matrix need the new name.

Drafted with Claude Code; reviewed and posted by me.