ci: run the full matrix automatically on the paths that break master (#281)

[pftg]

Closes the two actionable boxes on #281.

The matrix stays off PRs for free-tier minutes; the consequence is that the job which breaks master is a job that never ran on the PR. Three breakages this week, all invisible behind a green PR for exactly that reason:

breakage	path
Rails 7.1-only constant (#283)	test/
JRuby has no Kernel#fork (#283)	test/
zero-width skip_area mask (#280)	test/ + lib/

Every one was caught by adding full-ci by hand, after master was already red — a process that works only when someone remembers.

Change

The matrix now runs automatically when a PR touches test/, gemfiles/ or .github/. The label stays for everything else.

Changed paths are read from the API, not git diff — checkout is depth-1, so the base commit isn't in the clone to diff against.

Self-tested:

test/unit/foo_test.rb        -> FULL MATRIX
gemfiles/rails71_gems.rb     -> FULL MATRIX
.github/workflows/test.yml   -> FULL MATRIX
lib/snap_diff/config.rb      -> skip
README.md                    -> skip

lib/ is deliberately excluded

It changes on nearly every PR, and the functional + minimal-setup jobs already cover it. Including it would run 25 cells on almost everything and hand back the cost decision this exclusion exists to make. That is a trade, not a claim that lib/ is safe — and CONTRIBUTING.md now says exactly that, with guidance on when to add the label by hand.

Also documented in CONTRIBUTING.md

The two reading rules that cost real time this week:

• cancelled is not a pass. It occupies a verdict's slot while carrying none — the JRuby lane sat broken for 15 consecutive runs looking like this.
• gh run list --branch master needs --workflow Test. Without it you get whichever workflow ran last; it reported a Dependabot success while Test was failing on the same commit.

Still open on #281 and human-only: branch protection requiring the master-push matrix, once #249 settles the check names.

YAML validated.

Summary by Sourcery

Run the full CI matrix automatically for pull requests that modify historically high-risk paths while documenting the remaining opt-in and CI monitoring rules.

New Features:

• Automatically run the full Ruby and Rails test matrix for pull requests touching test/, gemfiles/, or .github/.

Enhancements:

• Retain manual full-ci opt-in for other pull requests while preserving the cost-conscious default.
• Document CI coverage boundaries, when to use full-ci, and how to interpret cancelled runs and verify the Test workflow on master.

CI:

• Detect pull request paths through the GitHub API to determine whether the full matrix is required.

Documentation:

• Add contributor guidance explaining automatic matrix triggers, manual opt-in scenarios, and reliable CI result checking.

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📥 Commits

Reviewing files that changed from the base of the PR and between d276752 and 25fad49.

📒 Files selected for processing (2)

• .github/workflows/test.yml
• CONTRIBUTING.md

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Reviewer's Guide

The PR makes the expensive CI matrix run automatically for pull requests touching test/, gemfiles/, or .github/ by querying changed files through the GitHub API, while retaining label-based opt-in and existing cost controls; contributor documentation explains the tradeoffs, manual opt-in cases, and reliable CI-result checks.

Flow diagram for automatic full CI matrix selection

flowchart TD
    A[Pull request event] --> B[GitHub API changed files]
    B --> C{Touches test/ gemfiles/ or .github/?}
    C -->|yes| D[Run full Test Ruby & Rails matrix]
    C -->|no| E{full-ci label present?}
    E -->|yes| D
    E -->|no| F[Run standard cost-controlled CI]
    G[Push to master manual dispatch or weekly drift check] --> D

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File-Level Changes

Change	Details	Files
Automatically opt pull requests into the full Ruby/Rails matrix when they modify historically high-risk paths, while preserving the existing cost controls for other PRs.	Add a pull-request path detector that queries the GitHub API with pagination and emits a job output for changes under test/, gemfiles/, or .github/. Use the detector output in the matrix job condition alongside master pushes, manual/scheduled runs, and the full-ci label. Keep lib/ excluded from automatic matrix runs and retain functional/minimal-setup coverage as the cost-conscious tradeoff.	.github/workflows/test.yml
Document when the full matrix runs and provide guidance for manually opting in and interpreting CI results.	Explain the automatic high-risk path triggers, the full-ci label fallback, and why lib/ is intentionally excluded. Warn that cancelled is not a passing verdict and provide the workflow-qualified command for checking the latest master Test run.	CONTRIBUTING.md

Possibly linked issues

• Master CI gatekeeper: a green PR does not predict a green master #281: The PR directly implements the issue's pending automatic full-CI path rule and documents its CI process.

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