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.

TODO.md

@@ -54,7 +54,7 @@
 - [ ] Consolidate PATH modifications into single, organized section in `commonrc.sh.tmpl`
 - [ ] Create modular PATH loading system (one block per tool with conditional checks)
 - [ ] Separate PATH, environment variables, and shell completions into logical sections
-- [ ] Document load order (`.bashrc` → `.zshrc` → `commonrc.sh` → `.bash_aliases`)
+- [ ] Document load order (`.bashrc` → `commonrc.sh` → `.bash_aliases`)
 - [ ] Add comments explaining each tool's PATH modification
 - [ ] Eliminate duplicate PATH additions
 - [ ] Create standard pattern for conditional tool loading
@@ -69,7 +69,7 @@
 - [ ] Move completions to dedicated section (after PATH, before aliases)
 - [ ] Add error handling for missing completion files
 - [ ] Document which tools provide completions
-- [ ] Test completions on bash and zsh
+- [ ] Test completions on bash
 - [ ] Add completion loading performance optimization
 - [ ] Document how to add new tool completions