chore: enable leanprover/skills plugin for Claude Code

This PR adds a `.claude/settings.json` that registers the
leanprover/skills plugin marketplace so Claude Code users working on
lean4 are prompted to install it. The plugin provides Agent Skills for
proof methodology, toolchain setup, bootstrapping, bisection, and PR
conventions.

Also un-ignores `.claude/settings.json` in `.gitignore` (the blanket
`settings.json` rule was blocking it).