fix(fish): activate hermit env on shell startup (#576)

## Problem

The `activate_hermit` handler in the fish hooks is registered only with
`--on-variable PWD`. In fish, that fires on *subsequent changes* to
`$PWD`, not on the value already set when the shell starts. Opening a
terminal directly into a hermit directory therefore never activates the
environment — you have to `cd` out and back in.

## Fix

Call `activate_hermit` once at the end of `fish_hooks.fish`. The hook
file is sourced from `~/.config/fish/conf.d/hermit.fish` on every shell
startup, so this handles the initial directory while `--on-variable PWD`
continues to handle `cd`.

This mirrors the existing shell hooks:
- **zsh** (`shell/zsh.go`): calls `change_hermit_env` immediately after
registering the `chpwd` hook.
- **bash** (`shell/bash.go`): runs `change_hermit_env` via
`PROMPT_COMMAND`, which fires before the first prompt too.

Fish was the only shell not activating on startup.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mightyguava

shell/files/fish_hooks.fish

@@ -35,3 +35,5 @@ function __complete_hermit
 end
 
 complete -f -c hermit -a "(__complete_hermit)"
+
+activate_hermit