fix(kernel): route the last abort linker through the one cascade

[drewstone]

Problem

#942 collapsed the supervision tree's abort cascade to one owner and reported "6 implementations → 1". The real count was 7. This copy survived because it is named linkSignals and lives in supervise/worktree-cli-executor.ts — a third name, in a file the search for mergeAbortSignals and abortIfSignalled never covered. I found it re-measuring after the merge, not before.

It carries the same defect #942 fixed: c.abort() with no argument on both branches, so a cascaded death through the worktree CLI executor still reaches the worker's down record naming no cause — the exact behaviour abort-reason.test.ts forbids.

Its docstring also claims it "returns undefined when neither is present"; the body has no path that returns undefined.

Change

The one call site uses linkAbort from supervise/abortable.ts. The copy and its stale docstring are deleted.

Why long-term right

Same reason as #942 — one cascade, one reason rule — and this PR is the evidence for it: a primitive with three names in four files cannot be found by searching for any one of them. With the last copy gone, grep -rn 'function linkSignals\|function mergeAbortSignals' src/ returns nothing.

Cost

1 file, net −16 lines. No public API change (check:version-bump: consumer surface unchanged).

Proof

• pnpm run typecheck, pnpm run lint (613 files), pnpm run docs:check — clean.
• tests/runtime/worktree-cli-executor.test.ts, tests/mcp/worktree-harness.test.ts, plus both abort suites — 52 passed.

Simplification

Simplification: the seventh and last hand-written abort cascade routed through the one owner; a docstring describing a return the function never makes, deleted with it.
Net: +2 / -18 lines, 1 file, 1 copy removed.
Not done here: none — this closes the primitive. abortError still has three factories and awaitAbortable two implementations, which #942 already named as its follow-up boundary and which remain out of scope for the same reason: collapsing them changes the message throwIfAborted produces at 135 call sites.
Tests: +0. The contract is already pinned on linkAbort by abort-reason.test.ts (re-pointed at production in #942); this PR moves a call site onto that owner and introduces no behaviour of its own to pin.

[tangletools]

✅ Auto-approved drewstone PR — 15b4e92f

This PR was opened by the trusted drewstone account.

This approval is provisional and was applied by the local stand-in because the pr-reviewer webhook host is unreachable (2026-08-21). CI on this head is fully green. The full PR reviewer audit re-runs via the resweep when the service returns and will publish findings if it detects issues.