Selection: a PR's e2e job runs only the tests its change reaches #347

Part of epic #337 (linked as parent). The owner approved the epic and all its batches on 2026-10-10, including this CI workflow change under clause 2; this child is batch 3 and inherits that approval.

User story

As the owner, I want a PR's CI to run only the feature tests its change reaches, so that PRs merge sooner.

Acceptance criteria

• On a PR, the e2e job runs the feature tests mapped to the changed code files' areas.
• The e2e job runs every test defined in a changed feature test file.
• A changed test file with an init, or a declaration another file uses, runs the whole feature package.
• A changed path in every-test runs the whole feature package.
• A changed code path with no area fails the e2e job before any test runs.
• A docs-only PR runs only TestFeatureMapCoverage and the coverage package tests.
• Selection reads each path's areas from both the base map and the head map.
• A map change that removes a test from any file's selection fails CI without map-narrowing-approved.
• Adding or removing a PR label reruns every CI job.
• A push to main runs the whole feature package.
• The job summary lists the affected areas, the selected tests and the count not run.

What I'd notice: a PR's e2e check runs part of the suite, and its summary reads "selected n of N".

Tasks

• Select, RunPattern, AlwaysRun in e2e/coverage/select.go.
• selecttests CLI in e2e/coverage/cmd/selecttests/main.go.
• .github/workflows/ci.yml: trigger types and the e2e job, as pinned in the contract.
• docs/testing.md: the selection section.
• The map-narrowing-approved label in relay.
• Tests from these criteria: select_test.go with fixtures in e2e/coverage/testdata/select/.

Contract

The planner's contract is the comment on #337, section 2 and child B. Clause 2: yes (ci.yml), approved by the owner on 2026-10-10.

Ownership:

• dev: .github/workflows/ci.yml (trigger types and the e2e job only), e2e/coverage/select.go, e2e/coverage/cmd/selecttests/main.go, docs/testing.md (selection section)
• test-writer: e2e/coverage/select_test.go, e2e/coverage/testdata/select/

Budget: 6 files, about 800 lines. Depends on child A (#343).

Waits: selection is a pure function of two commits; fixture repos are built with git in a temp dir.

Decisions made for the owner

• See the contract comment on Epic: PRs run only the e2e tests their change affects, with a full run at epic close #337.
• Ownership amendment (coordinator): CLAUDE.md (the e2e row of the What gates what table) joins this child's ownership, with the e2e row of the docs/testing.md gates table. Both are doc lines that would otherwise state the old run.
• Rule 2 does not treat an app build file as test-only; rule 1 counts any use of a changed file's top-level name; NARROWED lines print when the label is present. Details in the coordinator's comment.