`activate` is bash syntax, so fish users had no way to take on the dev
environment. `activate.fish` does what `activate` does: snapshots the keys it
will change, applies the composed env, defines a `hermes` function that
refuses outside the worktree, adds the prompt prefix, and defines
`deactivate`. It shells out to the same bash library, so composition is not
reimplemented.
`pm.environments --format fish` emits the env as `set -gx` statements.
fish's read-only names (`PWD`, `SHLVL`, `_`, ...) are skipped because
`set -gx PWD` aborts the script, and values named `*PATH` stay lists.
`hermes_compose_env` takes the dialect (`sh` or `fish`) as its second argument.
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