Add Repo object layer, fastgit.skill pyskill, and non-interactive git

[ncoop57]

Summary

Adds an object-oriented layer over the raw Git verbs, plus a pyskill documenting it for LLM agents.

• Repo: commits, refs, status, diffs, merges, rebases, stashes, blame, and remotes as live Python objects with terminal-style reprs (--oneline log, branch -vv, status -sb, --stat)
• Conflict is a status, not an error: every write op returns the resulting Status; conflicted entries expose .stages and :1:/:2:/:3: content for resolution
• b - a spells git diff a b; blame/trace share func=/lines=/regex= range keywords
• fastgit.skill registered as a pyskills entry point, including guidance that agents must never push (let alone force-push) without the user explicitly requesting that specific push
• callgit now runs git non-interactively (GIT_EDITOR/GIT_SEQUENCE_EDITOR suppressed): with captured output an editor can never function, so commands that would open one (e.g. rebase --continue after conflict resolution) previously hung forever; tested via git var GIT_EDITOR

Notes

• Docs in README/index regenerated from the notebooks.