Mutation gate: remove the 38 legacy Stryker directives on main with evidence-backed central exclusions #2060

Problem

Pull request #2051 forbids Stryker suppression comments going forward, but main still carries 38 legacy directives (42 comment lines across 9 files) that predate the policy. Per the policy, "suppressions left by earlier revisions are removed when their file is touched" — and the mutation gate's changed-code scope means these lines stay unmeasured until someone does.

The directives were never verified: several guard statements whose mutants existing tests already kill, so the suppression buys nothing.

Plan

Remove all 38 directives with per-line evidence, not bulk deletion:

1. Re-expose every previously-guarded statement to the real gate pipeline (buildManifest + runManifest, extension package) with the directives removed, in 3 batches.
2. Classify every surviving mutant: genuinely equivalent (accepted centrally) vs test gap (needs a real test).
3. Accept equivalents centrally in scripts/stryker-equivalences.json — the narrow gate-config exclusion the policy points to — keyed by file + line + mutator + replacement with a written rationale per entry, plus a stale-entry advisory so the registry cannot rot silently.

Expected behavior

• No // Stryker disable/enable comments remain in src.
• The gate reports the accepted equivalents in an Equivalent summary column and blocks on nothing else.
• Re-running the same repro on the cleanup branch yields blocking = 0 with no stale-entry advisories.

Scope

• 9 source files (comment-only removals): pathUtils.ts, abort-signal.ts, NativeToolCallParser.ts, webviewMessageHandler.ts, lm-studio.ts, qwen-code.ts, openai.ts, openai-codex.ts, Task.ts
• scripts/stryker-diff.mjs (equivalence matching + summary column) and scripts/stryker-equivalences.json (new registry)

Context

Step 2 of the two-PR plan in #2050 (enforcement shipped in #2051). Repro evidence and the full per-file disposition table are in #2054.