refactor(env): template environment.d config and update fish aliases

Replace hardcoded config/environment.d/generic.conf with a Tera
template (environments.tmpl) rendered via mise, pulling DOTFILES_DIR,
THEME_DIR, SECOND_BRAIN, and user info from mise.toml vars.

- Add SECOND_BRAIN variable in mise.toml for the vault path
- Update fish aliases to use $SECOND_BRAIN instead of a hardcoded path
- Fix Obsidian nvim aliases with proper quoting
- Remove stale dotfiles.conf template reference

templates/environments.tmpl

@@ -0,0 +1,8 @@
+TERM=xterm-256color
+npm_config_prefix=$HOME/.local
+
+DOTFILES_DIR = "{{ vars.DOTFILES_DIR }}"
+USER_FULLNAME = "{{ vars.USER_FULLNAME }}"
+USER_EMAIL = "{{ vars.USER_FULLNAME }}"
+THEME_DIR = "{{ vars.THEME_DIR }}"
+SECOND_BRAIN = "{{ vars.SECOND_BRAIN }}"