Retire the compatibility, section and writer Bats suites where Go covers them #924

Problem

Three smaller Bats suites under tests/skills/pr/ and tests/skills/writers/ also exercise behavior that children 01 to 03 and 11 now cover in Go: the compatibility declaration cases, the section-pipes cases (including the large-section-list regression), and the writer cases that run the description writers. After child 12 they test the shell twice. Only the writer suite's live probes are guarded by RUN_HEADLESS_WRITERS; its manifest permission, context-loading and workflow cases run without it.

Part of #588 and #323. Changes only those suite files. No shell body, shared helper or generated file changes.

Acceptance criteria

1. AC1: Given the compatibility, section-pipes and writer cases before the change, when the change lands, then a table in the pull request lists every case with its disposition: removed with the name of the Go test that covers it and the name of the retained frozen-path case that covers the same frozen-validator or frozen-writer outcome, or kept with the reason (including "frozen shell outcome with no other coverage: kept until the shell is removed").
2. AC2: Given the large-section-list regression cases (valid, missing, empty and reordered with 96 extra long sections), when the suite runs, then they are removed only because the Go test named for child 03 asserts the same four outcomes.
3. AC3: Given RUN_HEADLESS_WRITERS unset, when the writer suite runs, then its live probes are skipped as today while the manifest permission, context-loading and workflow cases still run; given it set, then the retained live probes also run; the flag is documented where it is read.
4. AC4: Given the frozen validator path, when the suite runs, then at least one compatibility case remains that runs validate-description.sh with a binary lacking the behavior, and no compatibility, section or writer case is removed whose only coverage of a supported frozen-validator or frozen-writer outcome (a finding, an exit status, a message) is itself.
5. AC5: Given the Bats jobs on Ubuntu and macOS, when they run on the merged head, then they pass.
6. AC6: Non-functional: the shared helper tests/skills/pr/test_helper.bash is not changed by this pull request.

Scope and non-goals

Scope: the three suites above and the table of dispositions.

Non-goal: the workflow suite (child 14) and the agent-pr suites with the helper trim (child 16). Non-goal: changing any shell. This pull request deletes only Bats cases: no script, library or generated copy is removed, and no owner-tier path in .agents/agent-skills.conf is touched (those go to the single cutover issue under #588). Non-goal: removing cases with no Go counterpart or whose frozen-shell outcome has no other retained coverage.

Approach

Meet AC1 to AC6 by building the disposition table first, resolving each Go test name to an existing test from children 01, 02, 03 and 11, then removing the covered cases in one pull request.

Dependencies

Blocked by: #921, #922.

Prerequisite: children 01 to 03 and 11 merged (reached through children 12 and 13).

Size

S. One pull request, mostly deletions, under the code line limit.

Verify plan

• AC1, AC2: the disposition table in the pull request, checked row by row at review, including the frozen-path coverage column.
• AC3: the writer suite run with the flag unset (asserting the permission, context-loading and workflow cases ran and the probes skipped) and set (asserting the probes ran).
• AC4: the retained case list, the table's frozen-path coverage column and the passing Bats job.
• AC5: the Bats jobs on Ubuntu and macOS on the merged head.
• AC6: the diff path listing shows no helper file.
• 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 removed case may cover an input the Go tests miss; the row-by-row review catches that. Undo: revert the change; the cases return.

Operator authority

None.