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_gitconfig.tmpl

@@ -27,7 +27,7 @@
 [user]
 	name = Xevion
 	email = xevion@xevion.dev
-	signingkey = C217005CF3C00672
+	signingkey = 301511AAD64FA365
 
 [commit]
 	gpgsign = true
@@ -45,6 +45,16 @@
 # Common configuration for all platforms
 [core]
 	editor = micro
+	pager = delta
+
+[interactive]
+	diffFilter = delta --color-only
+
+[delta]
+	navigate = true  # use n and N to move between diff sections
+
+[merge]
+	conflictStyle = zdiff3
 
 [init]
 	defaultBranch = master