dispatch: scope committed-read topology to enumerated repo

enumerateRepoCandidates resolved the committed-read topology (and thus the v1.1 mirror opt-in) via ResolveCommittedRefs(ctx), which loads settings relative to the context's worktree root — the process cwd unless overridden. When repoRoot differs from cwd (--repo/RepoPaths, or running outside any repo), the mirror opt-in was read from the wrong location, so v1.1 custom-ref reads could be missed or misapplied.

Derive repoCtx := settings.WithWorktreeRoot(ctx, repoRoot) and use it for SyncCommittedReadRef / NewCommittedReadStore. The GitStore reads are bound to the topology ref at construction, so only those two topology-resolving calls need the scoped context.

Add TestLocalMode_ExplicitRepoResolvesMirrorOptInFromTargetRepo, which opts the target repo into the mirror with its checkpoint only on the custom ref while cwd is a separate mirror-off repo. It fails without the fix (got []) and passes with it; the prior explicit-repo test read a v1-branch checkpoint that surfaced regardless of which settings applied.