`src/relay/git-handler-push-target.ts` and `src/main/git/remote.ts` carried
identical ~160-line copies of the resolver that decides which remote a plain
`git push` hits. Identical today is exactly when to share it: the cost of a
future divergence is pushing to the wrong remote, which retrying does not undo.
Move the resolver to src/shared/git-push-target-resolution.ts, parameterized on
a `(args) => Promise<{ stdout }>` runner — the only thing the two hosts actually
differ in — and delete both copies. The relay entry point keeps only the work
that is genuinely relay-side: re-validating an explicit target that arrived over
the wire and running `check-ref-format` on it.
No behavior change on either path, and nothing new or different is published, so
this engages no rule in remote-wire-compatibility. No git command changes.
src/relay/git-push-target-local-parity.test.ts scripts one repository's config
and requires `git.push` over the real relay dispatcher and the desktop's
`gitPush` to emit the same push argv, plus the argv each case should produce.
