fix: stricter GitHub owner/repo regexes and injection tests

Split _GITHUB_NAME_RE into separate owner and repo patterns.
Owner regex now rejects leading/trailing hyphens and dots (matching
GitHub's actual username rules). Repo regex requires alphanumeric
start but allows dots and underscores anywhere after.

New tests cover GraphQL injection attempts, invalid leading chars,
and valid hyphenated/underscore/dot combinations.

Co-Authored-By: Claude <[email protected]>