fix(coverage): probe the pwsh exemption with the executor's own resolution

The review caught the pwshCoverageExclusions comment claiming the inverse of
the code's behavior: a bare-PATH probe FAILING grants the exemption, so a
win32 host where only install-location pwsh (or 5.1) resolves kept the
exemption while its suites ran - over-exempting, never tightening.

resolvePwshPath/candidatePwshPaths move to the dependency-free
pwsh-local/src/resolve.ts (index.ts re-exports; public API unchanged) and
vitest.config.ts probes with that shared definition, so the exemption is
active exactly when the suites skip. The two spec headers stop saying
'on PATH' for a probe that is deliberately not PATH-only, and the parity
note records the abort backport as the one both-ways parity change (both
languages, pairing re-recorded); catalogs regenerated for the line shift.

packages/bash/tool-pwsh/tests/integration.spec.ts

@@ -4,7 +4,7 @@
  * process. These verify the world — actual commands run, stdout/stderr come
  * back, exit codes render, timeouts abort, background tasks settle through the
  * generic task runtime, and per-session cwd resolution works. The suite
- * self-skips when no `pwsh` is on PATH (a CI accommodation for hosts without
+ * self-skips when no usable `pwsh` resolves (a CI accommodation for hosts without
  * PowerShell); the fake-executor suite (tools.spec.ts) carries the coverage
  * gate.
  */