docs: document gated-marker coverage audit (#61)

[wjduenow]

Summary

• Adds a ## Pre-release coverage audit section to CONTRIBUTING.md — a one-shot command that runs all gated markers (bigquery, anthropic, cli_subprocess, e2e, wheel_smoke) under --cov in a single invocation, plus guidance on interpreting the default-vs-gated coverage delta.
• Cross-references the new section from .claude/rules/testing-signal.md § "Known gap: excluded markers".

Docs-only; no code paths touched. Addresses LOW finding L7 from the 2026-05-11 codebase review.

Closes #61

Test plan

• CONTRIBUTING.md § Pre-release coverage audit renders correctly
• Cross-reference in .claude/rules/testing-signal.md resolves

🤖 Generated with Claude Code

Summary by CodeRabbit

• Documentation
  • Added pre-release coverage audit guidance for maintainers, including instructions for running gated-marker tests with environment variables
  • Explained how to interpret coverage measurement differences before releases

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Pull request overview

This docs-only PR adds maintainer guidance for auditing coverage from pytest markers excluded from the default coverage badge, and cross-references that guidance from the testing rules.

Changes:

• Adds a pre-release gated-marker coverage audit section to CONTRIBUTING.md.
• Links .claude/rules/testing-signal.md to the new contributing guide section.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
CONTRIBUTING.md	Adds the gated-marker coverage audit command and interpretation guidance.
.claude/rules/testing-signal.md	Adds a cross-reference to the new pre-release audit documentation.

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[wjduenow]

PR Review Summary

Fixed (2 items)

File	Line	Issue	Commit
CONTRIBUTING.md	43	Command inherited --cov-fail-under=80 from addopts, so it could exit non-zero even when gated tests pass	3d33eba
CONTRIBUTING.md	49	Command produced standalone gated-only coverage, not a combined total — made the "additional coverage" delta non-reproducible	3d33eba

Both fixed in one change: the audit command now uses --cov-append (combines the gated run into the default run's .coverage file so the term report shows the combined total) and --cov-fail-under=0 (overrides the inherited 80% gate — this is a measurement, not a gate). Guidance rephrased to describe the combined-total delta.

False Positives (0 items)

None.