refactor(agents): remove the GitHub shell fallback after consumer migration #775

Problem

The GitHub subprocess migration in #760 retains the shell quota and identity readers for consumers pinned to older agentrepo binaries. Remove that fallback once all supported consumers provide the replacement.

Acceptance criteria

1. AC1: Given every supported consumer provides the GitHub replacement, the shell readers use it directly and refuse failures without fallback.
2. AC2: Given authentication, permission, quota and empty identity fixtures, statuses and diagnostic streams remain unchanged.
3. AC3: Generated skills match their canonical source and check-generated-skills passes.

Related

Deferred from #760; part of #588. Removal waits for the supported binary floor.