Warn when an extraRepos entry cannot track a repository

[MPIsaac-Per]

What was wrong

discoverRepos parsed the extraRepos setting by filtering the split entries through isRepoName and keeping the survivors. Anything that failed the test had no else branch: no warning, no error, no record. A wildcard such as owner/*, or a plain typo, therefore behaved exactly like a setting that was working and had simply found nothing, and the only way to discover the difference was to read the regex.

Fixes #2540

What changed

• plugins/github/server.ts: added an exported parseExtraRepos that splits the setting into valid and invalid entries instead of filtering in place. discoverRepos tracks the valid ones as before and emits bb.log.warn for each invalid one, naming the entry and the expected owner/repo shape.

No wire, CLI, or settings-schema changes. extraRepos keeps its meaning and its type; wildcards remain unsupported, they are just no longer silent about it.

How you verified

• plugins/github/server.test.ts: a unit test over parseExtraRepos covering a wildcard, a typo, a mixed list, and blank or separator-only input.

• plugins/github/server.rpc.test.ts: a behavioral test asserting that with extraRepos: "acme/widgets, ACME/*" the valid sibling still syncs one repo and a warn entry naming ACME/* reaches harness.logEntries.

• Both fail before this change and pass after. Reverting server.ts alone: 25 passed, 2 failed. With the change: 27 passed.

• pnpm exec turbo run typecheck test --filter=bb-plugin-github passes.

• Manually on a dev instance at this commit: extraRepos = "MPIV-AI/*, mpiv-ai/clipscript" now logs

  WARN: [server] [plugin:github] extraRepos entry "MPIV-AI/*" is not an owner/repo name and tracks nothing. List each repository explicitly, for example "owner/repo".
  

  while mpiv-ai/clipscript beside it still tracks and syncs.

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