Unused shared Bats helpers are removed #885

Problem

Children 15 to 19 retire Bats cases but leave the shared helpers other suites may still load: the two test_helper.bash files and gh-merge.bash, 299 code lines in tests/skills/agent-merge-pr and tests/skills/merge-pr. Once every merge suite is retired and the fallback is unreachable (child 20), helpers nothing loads are dead code, and no earlier child owns them.

Production-library work is not in this child: the merge-only parts of packages/agents/source/lib/lib.sh match constitutional_paths and owner_paths in .agents/agent-skills.conf at origin/main, so any such removal belongs to the single cutover issue under #588.

Part of #324, within #588. Child 24 in #324's Children list. Removes unused Bats helpers under tests/skills/agent-merge-pr and tests/skills/merge-pr.

Acceptance criteria

1. AC1: Given a search of both test families (tests/skills and tests/hooks) for each helper file and function, when it runs, then every helper that remains is loaded by at least one remaining suite, and the pull request lists each removed helper with the search that showed it unused.
2. AC2: Given mise run test-hooks, when it runs after the removal, then every remaining suite passes.
3. AC3: Given the merged change, when its file list is read, then it contains only files under tests/, and no path matched by constitutional_paths or owner_paths in .agents/agent-skills.conf.
4. AC4: Non-functional: the Bats suite wall time does not increase.

Scope and non-goals

Scope: unused Bats helpers only.

Non-goal: removing helpers any remaining suite loads. Non-goal: production code, including the merge-only parts of lib.sh (cutover issue under #588), and Go tests.

Approach

Meet AC1 to AC4 by searching both test families for each helper, removing the unused ones and running the suite.

Dependencies

Blocked by: #877, #878, #879, #880, #881, #882.

Prerequisites: none.

Size

XS. One pull request, deletions only: the three helper files total 299 code lines at origin/main (a physical-line code count).

Verify plan

• AC1: the per-helper search output in the pull request.
• AC2: mise run test-hooks at the merged head.
• AC3: the pull request's file list and the protected-path scan output.
• AC4: Bats wall times before and after.
• Definition of done: D1 each AC is verified at the merged head, with the commands and results cited in the pull request's Verification. D2 an independent review approves the exact merged head, and every finding has a recorded disposition. D3 required checks pass with no gate loosened. D4 the leak scans the repository specifies are clean, and every post made for this work passes the per-post leak review before it is posted. D5 every commit is signed. D6 the documentation matches the change or the Verification says why there is no impact. D7 every commit has an Assisted-by trailer and the pull request has the provenance:agent-assisted label. D8 the pull request stays within the size limits. D9 follow-ups are filed as issues and listed under Related. D10 the merge closes the issue through a closing reference, the board shows Done, and the final Status: comment names the merge commit.

Risks

A helper loaded dynamically would be missed by a search and break a suite; AC2 runs every suite. Undo: revert.

Operator authority

None.