docs: explain autonomous-science metric definitions and exclusions

[fraware]

Why this helps

sm metrics autonomous-science already emits numerator/denominator/included_ids/exclusions for each assurance slice, but docs/metrics.md only has a one-paragraph summary. New contributors (and reviewers of Gate 6 assurance failures) need a readable definition of what each metric counts, what exclusion reasons mean, and what the numbers do not claim.

Scope

Add a dedicated subsection (or short companion page under docs/assurance/) that documents every key in METRIC_KEYS from pipeline/src/sm_pipeline/assurance/metrics/__init__.py, matching actual slice construction in that module. Cross-link from docs/metrics.md and docs/assurance/README.md.

Suggested starting points

• pipeline/src/sm_pipeline/assurance/metrics/__init__.py (METRIC_KEYS, _slice, compute_autonomous_science_metrics)
• docs/metrics.md (Autonomous science paragraph)
• docs/assurance/README.md, docs/assurance/pilot-interpretation.md
• benchmarks/assurance/README.md, benchmarks/baseline_thresholds.json (floors context)
• examples/assurance-pilot/metrics.json (example output shape)

Acceptance criteria

• Each metric key has: plain-language definition, what enters the numerator, population/denominator, and typical exclusion reason strings
• Explicit non-claims block (not live authorization rates; does not rewrite claim status; presence of a record is not acceptance)
• Links from docs/metrics.md and docs/assurance/README.md
• Example walkthrough using pilot fixtures / examples/assurance-pilot/metrics.json
• No schema or CLI behavior changes required for this issue

Non-goals

• Changing metric formulas, floors, or Gate 6 scorers
• Building a portal metrics UI (separate issue if desired)
• Rewriting PCS or corpus claim semantics

How to test

uv run --project pipeline sm metrics autonomous-science --out /tmp/assurance-metrics.json
# Manually confirm the new docs match keys and exclusion reasons in the JSON
# Spot-check against examples/assurance-pilot/metrics.json after regenerating if needed:
uv run --project pipeline sm import-assurance-release tests/assurance/fixtures/pilot-retrospective/
uv run --project pipeline sm metrics autonomous-science --out examples/assurance-pilot/metrics.json

If labels good first issue / help wanted are unavailable in your fork, ask a maintainer to apply them; they exist on fraware/scientific-memory.