C25c: CLI dogfood quick wins (B1+B2+B4)

[espadonne]

Summary

Bundles the three CLI-side findings from the B-audit. Each is independently small; ships as one PR for review efficiency (matches the S62 envelope-batch pattern).

• B1 — pr merge success line accepts merge_commit_sha. MergeResult now decodes both the gh-compat field and the legacy sha. New accessor CommitSHA() prefers gh-compat. Without this the success line was rendering ("Merged PR #N at ") with an empty SHA on shithub builds that emit only merge_commit_sha.
• B2 — --json field gh-compat aliases. repo view / repo list export catalogs expose forkCount, stargazerCount, watcherCount alongside the legacy forks / stargazers / watchers. Ported gh scripts (gh repo view --json stargazerCount) work verbatim. Both names populated for one release cycle.
• B4 — repo fork friendlier own-repo error. Pre-flight check resolves the authenticated user (cached) and refuses with a clear message naming the repo before round-tripping. --org bypasses the guard (forking your own repo into a different org is a real workflow).

Test plan

• TestMergeCommitSHA_GhCompat — decoder accepts merge_commit_sha.
• TestMergeCommitSHA_PrefersGhCompat — gh-compat wins on both-fields-set.
• TestViewJSONGhCompatAliases — --json stargazerCount,forkCount,watcherCount projects numeric values.
• TestForkRejectsOwnRepo — clear pre-flight error; forks endpoint never hit.
• TestForkOwnRepoIntoOrgAllowed — --org bypass.
• Full make ci green locally.