feat: migrate from Zsh to Fish shell with comprehensive configuration

Major shell migration replacing Zsh/Oh-My-Zsh with Fish shell:

- Remove all Zsh configurations (dot_zshrc.tmpl, dot_p10k.zsh)
- Remove Oh-My-Zsh external dependencies from .chezmoiexternal.toml
- Add complete Fish shell setup with config.fish.tmpl and abbr.fish.tmpl
- Implement Fish-native functions for lazy-loading tools (mise, pyenv, zoxide, etc.)
- Create commonrc.fish.tmpl for cross-shell compatibility
- Add Fish plugin management via Fisher (tide prompt, fzf.fish)
- Update documentation (CLAUDE.md, TODO.md, ONBOARDING.md) to reflect Fish
- Add .fish.tmpl file association to VS Code settings
- Enhance PowerShell profile with lsd aliases
- Configure git delta pager and zdiff3 merge conflict style
- Update WSL keychain integration for Fish shell

This migration maintains all existing tool integrations while improving
startup performance through lazy-loading and Fish's native features.

home/dot_config/fish/functions/zi.fish

@@ -0,0 +1,18 @@
+# Lazy-load zoxide on first use of zi command
+function zi --description "zoxide interactive smart cd"
+    # Initialize zoxide only once (shared flag with z.fish)
+    if not set -q __zoxide_initialized
+        set -g __zoxide_initialized 1
+
+        # Run zoxide init
+        if command -q zoxide
+            zoxide init fish | source
+        else
+            echo "zoxide is not installed" >&2
+            return 1
+        end
+    end
+
+    # Execute the actual zi command (defined by zoxide init)
+    __zoxide_zi $argv
+end