chore(harness-config): drop the audit's one-release .claude/source-control.md lane read #13473

Refs #9737

Outcome

/harness-config:audit detects a declared unattended-push lane (narrowing 1) only from docs/conventions/source-control.yaml. It no longer falls back to the ## babysit_loop_merge H2 of <repo>/.claude/source-control.md. That fallback has been read for one release since harness-config 2.5.0 (plugins/harness-config/CHANGELOG.md:145-151), and 3.0.13 has shipped since. #13042 retired the file for the source-control plugin but left this reader in place.

Change

• plugins/harness-config/skills/audit/scripts/audit-engine.sh:1455-1510:
  • Remove SC_TEAM_REL/SC_TEAM, lane_read_legacy and the markdown awk block.
  • Set :1510 to lane_file="$SC_TEAM_YAML_REL": under set -uo pipefail (:89) a leftover $SC_TEAM_REL is an unbound variable.
  • Keep the YAML read and its rule that a YAML the shared parser rejects gives no signal.
  • Update the comment at :1455-1465.
• plugins/harness-config/skills/audit/scripts/audit-engine.test.sh, cases near :1656-1745 (case 49 and its neighbours):
  • Move each fixture that declares the lane in .claude/source-control.md to docs/conventions/source-control.yaml.
  • Keep one case asserting that a lane declared only in the old file gives no signal.
  • Delete the markdown-parser-only cases (:1733 CRLF and backticks, :1745 padded value) rather than moving them.
• plugins/harness-config/skills/audit/reference/audit-checklist.md:30 and plugins/harness-config/skills/audit/reference/required-permissions.md:306: name docs/conventions/source-control.yaml as the team-tracked file.
• Add a harness-config changelog fragment.

Done when

• git grep -nP '(?<!~/)(?<!HOME/)(?<!\}/)\.claude/source-control\.md' -- plugins/harness-config ':!*CHANGELOG*' ':!*.test.sh' prints nothing (on origin/main it prints 4 lines in 3 files).
• grep -n '^## \[3.0.0\]' plugins/harness-config/CHANGELOG.md prints one line (a release after the 2.5.0 reader).
• bash scripts/run-test-file.sh plugins/harness-config/skills/audit/scripts/audit-engine.test.sh exits 0.
• scripts/check-changelog-fragments.sh --check-required origin/main exits 0.