refactor(agents): remove the gh-post shell adapter after consumer migration #777

Problem

The port in #756 keeps the shell gh-post adapter for consumers whose agentrepo binary lacks the posting capability. Once every supported consumer provides that capability, the duplicate implementation can go.

Acceptance criteria

1. AC1: Given every supported consumer provides the posting capability, the requirement check and posting adapter call Go directly and never use a shell fallback.
2. AC2: Missing-tool refusal, argument and stdin forwarding, separate output streams, exit statuses, and silent requirement checks keep their existing results. Supported failures make no second attempt.
3. AC3: Generated skill output matches its canonical source, and mise run check-generated-skills passes.

Dependencies

#756 and migration of every supported consumer to a binary with the posting capability.

Related

Deferred from #756; part of #588. Removal waits for the supported consumer migration.