Move git identity to an included ~/.gitconfig.local

~/.extra ran five `git config --global` writes on every shell startup, to
re-assert identity and signing after bootstrap.sh copies gitconfig over
~/.gitconfig. The dark-mode-notify agent fires on wake/unlock and runs
`pkill -usr1 zsh`, so every shell re-sources .zshrc — and therefore .extra —
simultaneously. `git config` locks with O_EXCL and has no retry or timeout for
config files, so the concurrent writes raced and printed:

    error: could not lock config file /Users/jonny/.gitconfig: File exists

Identity and signing now live in an untracked ~/.gitconfig.local, included
last from gitconfig so it wins over anything above. bootstrap.sh seeds it from
gitconfig.local.template only when absent, so re-runs never clobber the real
signing key. No git writes happen on shell startup at all now.

Verified: fully-resolved config is unchanged apart from the new include.path;
author and committer both resolve from config with no GIT_* env vars set;
existing commits still verify. 12 concurrent sources of .extra are silent,
where the old version produced 27 lock errors.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

gitconfig

@@ -87,3 +87,9 @@
     autoSquash = true
     autoStash = true
     updateRefs = true
+
+# Machine-local identity and signing config. Untracked, and included last so
+# it wins over anything above. Keeps `bootstrap.sh`'s copy of this file
+# non-destructive without needing to re-apply values on every shell startup.
+[include]
+    path = ~/.gitconfig.local