refactor(source-control): read the commit and PR convention keys from docs/conventions/source-control.yaml #9725

Parent

Refs #5906

Depends on

• none: on the frontier now

Outcome

The source-control plugin's commit and PR convention keys (pr_body_required_sections and the per-key
convention keys config-resolution.md defines) resolve from docs/conventions/source-control.yaml, which
already holds pr_open_state; .claude/source-control.md is read only when the YAML lacks the key, for one
release. /source-control:setup writes the YAML.

Decisions

• ADR 0064 Decision 1: a plugin's team settings live in one docs/conventions/<concern>.yaml, validated by a
  JSON Schema at plugins/<plugin>/schemas/<concern>.schema.json (Decision 6), checked in CI by
  scripts/check-convention-yaml.sh (Decision 7). Script readers use the shared reader lib/parse-concern-value.sh.
• ADR 0075: layers are plugin default < userConfig < team <home>/<concern>.yaml < personal
  <home>/<concern>.local.<ext>; every resolver reports the layer that supplied each value.
• Migrate per-skill .claude config files to userConfig, one conventions doc, or the cascade (ADR 0018) #5906 (comment 2026-10-04): each reader keeps reading the old file for one release when the new file is
  absent, and this repository keeps its old file until the cleanup slice.

Key interfaces

• plugins/source-control/schemas/source-control.schema.json, plugins/source-control/reference/config-resolution.md,
  the commit, pull-request and setup skills, plugins/source-control/skills/pull-request/scripts/parse-branch-issue.sh.
• harness-config's audit engine mentions of the old file (plugins/harness-config/skills/audit/).

Done when

• docs/conventions/source-control.yaml holds pr_body_required_sections with this repository's four sections, and scripts/check-convention-yaml.sh --check exits 0.
• scripts/run-test-file.sh plugins/source-control/lib/config-root.test.sh and scripts/run-test-file.sh plugins/source-control/skills/pull-request/scripts/parse-branch-issue.test.sh exit 0, with cases for: the YAML value wins, the old file is read when the YAML lacks the key, and the resolver names the layer that supplied it.
• scripts/run-test-file.sh plugins/harness-config/skills/audit/scripts/audit-engine.test.sh exits 0.
• git grep -n ".claude/source-control.md" -- plugins/source-control/skills/commit plugins/source-control/skills/pull-request plugins/source-control/skills/setup prints only compatibility-read lines.
• The PR body lists each migrated key with its new home and schema.
• scripts/check-changelog-fragments.sh --check-required origin/main exits 0.

Out of scope

• The babysit_loop_* keys and the guardrails commit-convention resolver (their own slices under Migrate per-skill .claude config files to userConfig, one conventions doc, or the cascade (ADR 0018) #5906).
• Deleting .claude/source-control.md.