feat(sunrise): print the PATH hint for the user's login shell

- new.sh reads the basename of SHELL (via ${SHELL:-} under set -u) and
  prints fish_add_path "<dir>" for fish, which keeps it for new sessions,
  export PATH="<dir>:$PATH" naming ~/.zshrc or ~/.bashrc for zsh and
  bash, and today's generic export line with "your shell profile" for
  any other, empty or unset shell
- dq_escape escapes the install directory for the target shell's double
  quotes: backslash, double quote and dollar always, plus backtick and a
  single-quoted ! for POSIX shells; a per-character loop, so bash 3.2
  and 5.2 behave the same
- the behaviour harness defaults SHELL to empty so cases do not depend on
  the developer's login shell, adds an unset-SHELL mode and covers fish,
  zsh, bash, empty, unset, unknown and two metacharacter directories
- TestBootstrapEndToEnd pins SHELL=/bin/bash
- sunrise README and docs/setup/installation.md describe the per-shell
  hint; the manual PATH section gains fish_add_path (go env GOPATH)/bin

docs/setup/installation.md

@@ -30,7 +30,7 @@ Run the one-liner from the directory that should hold both the framework checkou
 
 1. It checks the prerequisites. Go older than 1.27 or missing, or a missing git, stops the script with an install hint before anything is cloned. A missing Docker or Docker Compose plugin is only a warning, because the application needs them to run its database, not to be created.
 2. It clones the framework into `./summercms` and checks out the newest `vX.Y.Z` release tag, detached. When `./summercms` already exists, the script fetches tags and moves the checkout forward to the newest release only when that is a fast-forward. It refuses, without changing anything, a directory that is not a framework checkout, a checkout with changes to tracked files and a checkout that is on a branch.
-3. It installs the `summer` CLI from that checkout with `go install ./cmd/summer`. When the Go install directory is not on your `PATH`, the script ends with the `export PATH=...` line to run and to add to your shell profile.
+3. It installs the `summer` CLI from that checkout with `go install ./cmd/summer`. When the Go install directory is not on your `PATH`, the script reads your login shell from `SHELL` and ends with `fish_add_path <dir>` in fish, which keeps the directory for new sessions, or with `export PATH="<dir>:$PATH"`, to add to `~/.zshrc` in zsh, `~/.bashrc` in bash or your shell profile in any other shell.
 4. It runs `summer new demo` next to the checkout, with `--framework` pointing at it, which writes `sm-demo-app` and prints the next steps. The script itself ends with the `cd` line into the new application.
 
 The script stops before cloning when the name is invalid or `sm-demo-app` already exists. Three environment variables change its defaults:
@@ -99,12 +99,18 @@ go install ./cmd/summer
 summer --help
 ```
 
-`go install` writes the binary to `$(go env GOPATH)/bin`, usually `~/go/bin`. If `summer --help` reports `command not found`, that directory is not on your `PATH`. Add it for the current shell, and to your shell profile to keep it:
+`go install` writes the binary to `$(go env GOPATH)/bin`, usually `~/go/bin`. If `summer --help` reports `command not found`, that directory is not on your `PATH`. In bash or zsh, add it for the current shell, and to `~/.bashrc` or `~/.zshrc` to keep it:
 
 ```sh
 export PATH="$(go env GOPATH)/bin:$PATH"
 ```
 
+In fish, `fish_add_path` adds it and keeps it for new sessions without a profile edit:
+
+```sh
+fish_add_path (go env GOPATH)/bin
+```
+
 The tool creates, builds and watches applications, scaffolds plugins, models, migrations and admin controllers, and builds this documentation. Check that the framework compiles and its unit tests pass:
 
 ```sh