fix(github): report extraRepos entries that are not owner/repo

[sujeito-operator]

What was wrong

discoverRepos filtered the extraRepos setting through isRepoName and dropped whatever failed, with no log line and no mention on any surface. A wildcard like SOME-ORG/*, a bare owner or a typo was stored by the settings form and then ignored, which is indistinguishable from a setting that worked and matched nothing — exactly what #2540 reports.

What changed

plugins/github/server.ts:

• New parseExtraRepos(raw) returns { repos, ignored } — the names to track and the entries that are not owner/repo. Separators and blanks are structure, not entries, so an empty setting has nothing to complain about.
• discoverRepos warns via bb.log.warn once per distinct ignored set, not per discovery: it re-reads settings whenever the 60s cache lapses, so warning per pass would repeat the line forever behind the background sync.
• bb github repos names the ignored entries on stderr — the surface the report actually used. stdout is unchanged, so the repo list stays parseable.

Wildcards are still not supported. This makes the refusal audible rather than adding the feature; the whole-owner/selection ask is #2543.

plugins/github/README.md documents the behaviour in the same change.

Not done deliberately: the settings form itself does not show this. PluginSettingDescriptor is documented as "deliberately plain data — not zod — so the host can render settings forms and the CLI can parse values without executing plugin code", so there is no plugin-side validation hook. Adding one would be a new public plugin API surface (experimental_ prefix, docs/api_to_audit.md entry, a surfaces.ts card) and is a much larger change than this bug warrants.

No wire change, so no HOST_DAEMON_PROTOCOL_VERSION bump. No new CLI command or flag — existing bb github repos gains diagnostic stderr only.

How you verified

• server.test.ts — separates usable extraRepos entries from ones it cannot honor: unit cover for the parser, including the SOME-ORG/* wildcard from the report, the get-bb/bb, nonsense typo case, blanks/duplicate separators, and dedupe.
• server.rpc.test.ts — reports extraRepos entries it cannot honor instead of dropping them: drives the real plugin through runCli(["repos"]) and asserts both the stderr line and the logEntries warning, then calls repos a second time and asserts the warning was emitted once, not per discovery.
• server.rpc.test.ts — says nothing about extraRepos when every entry is usable: the silence case, so the warning cannot regress into noise.

Both new tests fail on main and pass with this change (verified by reverting server.ts alone and re-running: 2 failed / 4 passed → 6 passed).

turbo run test --filter=bb-plugin-github: 6 files passed. turbo run typecheck --filter=bb-plugin-github: clean.

Fixes #2540

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Disclosure: this patch was written and tested end to end by an autonomous AI agent; a human principal is accountable for it. What this account is. Ask me anything about how it was produced and I will answer.

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