[ZSH] Faster startup: instant prompt, single compinit, cached probes

Re-enable p10k instant prompt (sudo-key ssh-add moved above it). Drop
oh-my-zsh.sh (second compinit, update check); set up what OMZ plugins
need ourselves. Cache java_home, go env, fzf --zsh output. Disable the
kubecontext segment (ran kubectl per shell). Log starts over 1 s to
~/.cache/zsh/startup-slow.log to catch cold-start stalls.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

zsh/.mac_config.zsh_mac.example

@@ -1,6 +1,8 @@
 export LC_ALL=en_US.UTF-8
 export LANG=en_US.UTF-8
-export JAVA_HOME=`/usr/libexec/java_home`
+# deps: system/user JDK dirs, which change when a JDK is added or removed
+_zsh_cached java_home /Library/Java/JavaVirtualMachines ~/Library/Java/JavaVirtualMachines \
+    -- /usr/libexec/java_home && read -r JAVA_HOME < $REPLY && export JAVA_HOME
 # macOS resets this on updates (and at random); background it, it's slow
 defaults write .GlobalPreferences AppleLocale en_DE &!