Show and verify commit signature status in gg

[faun]

Why

gg grew a %G? column so each unmerged commit's signature state sits next to its hash. That column could not work as shipped: commits here are ssh-signed through 1Password, and git cannot attempt verification of an ssh signature without gpg.ssh.allowedSignersFile. So every gg run printed error: gpg.ssh.allowedSignersFile needs to be configured and exist for ssh signature verification once per commit and reported every commit as N, which is exactly what a genuinely unsigned commit reports. The column was worse than useless, it was misleading.

What changed

• install/01_configure_git_signing.sh (new): generates ~/.config/git/allowed_signers from the ssh signing keys registered on GitHub. Writes via mktemp + mv, so a failed or empty API response leaves an existing file untouched, and skips with a message when gh, user.email, or github.user is missing. Re-run with ./install.sh --force 01_configure_git_signing after registering a new key.
• config/git/config: points gpg.ssh.allowedSignersFile at that file. Side benefit: git verify-commit, git log --show-signature, and git merge --verify-signatures start working too.
• shrc/git.sh: new git_can_verify_signatures helper; gg interpolates the %G? token only when verification is actually possible, so a machine without the signers file shows no column rather than a column of misleading Ns.

Keys come from GitHub rather than ~/.ssh because history carries four signing keys from different machines and GitHub has all four. Reading the local key alone would leave anything signed elsewhere reading U. The generated file stays out of this repo on purpose: the repo is public, and the moment it grew a colleague's key it would be publishing their address. Other people's commits reading U is the correct outcome.

Verification

Check	Result
./install.sh 01_configure_git_signing	Wrote 4 signing keys
git verify-commit HEAD	Good "git" signature for git@faun.me ..., exit 0
%G? over last 300 commits	267 G, 33 N (genuinely unsigned), 0 U, 0 error: lines
gg on a branch with unmerged commits	* 44a439d G Faun: Split operations guide..., no errors
Guard, signers file moved aside	helper returns 1, gg prints no column and no error
Failure path (bogus github.user)	404 handled, existing file intact
zsh test/warp_test.zsh	27 passed, 0 failed

Confirmed along the way that verification works despite gpg.ssh.program = op-ssh-sign: git picks the verification backend from each signature's own payload rather than from gpg.format, and uses ssh-keygen for ssh signatures.