Add standalone Git identity tools

[lox]

Summary

• add standalone git-assume for shells and agents, including unsigned identities without signing keys
• add git-github-auth to switch repository-local GitHub HTTPS/SSH auth rewrites
• update Git identity docs and agent commit guidance
• remove the global lox GitHub SSH rewrite in favour of the per-repo helper

Checks

• bash -n bin/git-assume bin/git-github-auth
• shfmt -d -i 2 -ci -bn -s bin/git-assume bin/git-github-auth
• git diff --check
• temp-repo functional checks for git-assume and git-github-auth

Note: shellcheck and pre-commit were not installed locally.