chore(tools, github): Add repository, page params + comment fetching

[JeanMertz]

The github_issues and github_pulls tools were previously hardcoded to the project's own GitHub repository and returned no comments. This commit addresses both limitations.

Both tools now accept a repository parameter in owner/repo form, defaulting to the project's own repo when omitted. A parse_repo() helper centralises the parsing and default logic, replacing ad-hoc split_once('/') calls scattered across the tool modules.

A page parameter was added to both tools so that long comment threads can be walked incrementally. When a specific number is provided, the tool fetches that page of conversation comments (10 per page) and includes them in the response alongside a comments_count total. The page size is intentionally bounded to keep responses within LLM context limits.

The underlying jp_github crate gained a list_comments() method on IssuesHandler, backed by a new IssueCommentListBuilder and a Comment model. The Issue model also now carries a comments field (the total count) which GitHub exposes on both the list and detail endpoints.

Auth handling was also improved: github_issues, github_pulls, and github_read_file now use a new auth_optional() path that falls back to anonymous access when no token is configured. Read-only operations against public repos no longer require a token.

Finally, web_fetch now detects GitHub issue and PR URLs and returns a redirect error pointing at the correct dedicated tool, rather than silently returning near-empty HTML (GitHub renders comments client-side).