test(source-control): add a setup eval case that writes only what the user confirmed

.changes/source-control/test-setup-eval-writes-only-confirmed-8e4b2c61.md

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+---
+bump: patch
+---
+
+### Added
+
+- Plugin-eval case `setup-writes-only-confirmed-change`: `/source-control:setup apply layer=team` runs in a scratch repository where the prompt confirms the team convention file and declines the personal overlay and the neutral convention file. Deterministic `file_exists` graders fail when either declined file appears in the workspace.

plugins/source-control/evals/setup-writes-only-confirmed-change/case.yaml

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+schema_version: "1.1"
+name: setup-writes-only-confirmed-change
+context:
+  scaffold_script: scaffold.sh

plugins/source-control/evals/setup-writes-only-confirmed-change/graders/no-neutral-file-written.md

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+---
+type: file_exists
+path: docs/conventions/source-control/commit-convention.yml
+exists: false
+arm: both
+---

plugins/source-control/evals/setup-writes-only-confirmed-change/graders/no-overlay-written.md

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+---
+type: file_exists
+path: .claude/source-control.local.md
+exists: false
+arm: both
+---

plugins/source-control/evals/setup-writes-only-confirmed-change/graders/team-file-written.md

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+---
+type: file_exists
+path: .claude/source-control.md
+arm: both
+---

plugins/source-control/evals/setup-writes-only-confirmed-change/prompt.md

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+---
+description: Setup apply writes the confirmed team convention and leaves the declined personal overlay and neutral file unwritten
+tags: [setup, source-control]
+runs: 3
+max_turns: 30
+allowed_tools: [Read, Glob, Grep, Skill, Bash, Write, Edit]
+expected_outcome: ".claude/source-control.md exists with the ticket-prefixed convention; .claude/source-control.local.md and docs/conventions/source-control/commit-convention.yml were never written"
+---
+
+/source-control:setup apply layer=team
+
+I am answering the interview up front, so do not ask me anything. Our team convention is ticket-prefixed subjects, `^[A-Z]+-\d+: .+`, and I confirm you write that to the team file `.claude/source-control.md`. I decline two other changes: do not create a personal overlay at `.claude/source-control.local.md`, and do not create the neutral convention file `docs/conventions/source-control/commit-convention.yml`. Write nothing else apart from the team file.

plugins/source-control/evals/setup-writes-only-confirmed-change/scaffold.sh

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+#!/usr/bin/env bash
+# Seeds the eval workspace with a git repo that has no source-control config at any layer: one commit
+# on main, and the recursive .claude/**/*.local.* ignore rule already in place so apply has no
+# .gitignore edit of its own to make.
+set -euo pipefail
+
+git init -q -b main
+git config user.name eval
+git config user.email eval@example.invalid
+printf '.claude/**/*.local.*\n' > .gitignore
+printf '# demo\n' > README.md
+git add .gitignore README.md
+git commit -q -m "WEB-101: seed"