fix: name an absolute path in the sudo re-run hint

The firecracker setup message told users to run

    sudo bot-bottle backend setup --backend=firecracker

which fails for exactly the users who followed the documented install. sudo
replaces PATH with sudoers' secure_path — /usr/local/sbin:/usr/local/bin:
/usr/sbin:/usr/bin:/sbin:/bin on Debian and Ubuntu — which deliberately
excludes user-writable directories. Both supported install paths land in one:
pipx uses ~/.local/bin, and install.sh's venv fallback symlinks there. So the
hint works for anyone who installed system-wide and breaks with "command not
found" for everyone else, which is how it survives a read-through.

Add bot_bottle/invocation.py: self_path() resolves the running entry point to
an absolute path, and sudo_command() builds the copy-pasteable form. The one
sudo recommendation in the tree now uses it. Non-sudo hints keep the bare
`bot-bottle`, which is correct — the user reached them by running it.

self_path() falls back to the bare name when argv[0] cannot be resolved (a
`python -m` style invocation), because a slightly wrong hint beats a traceback
raised while reporting some unrelated problem.

Tested behaviourally rather than by scanning source: the first version of the
test grepped the module and failed on the comment explaining why the bare form
is wrong. It now drives _setup_systemd() as non-root and asserts what is
actually printed. Verified the guard bites by restoring the bare form and
watching it fail.

Not verified end to end: this message only prints on the systemd path, so it
is unreachable on macOS, where the rest of this work was tested.

bot_bottle/invocation.py

@@ -0,0 +1,48 @@
+"""How to tell a user to re-run this CLI.
+
+`bot-bottle …` is the right thing to print for anything the user runs as
+themselves — it is on their PATH, since that is how they got here.
+
+Under `sudo` it is not. sudo replaces PATH with sudoers' `secure_path`
+(`/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin` on Debian and
+Ubuntu, similar elsewhere), which deliberately excludes user-writable
+directories. Both supported install paths put the entry point in one of those:
+pipx uses `~/.local/bin`, and `install.sh`'s venv fallback symlinks there too.
+So `sudo bot-bottle …` fails with "command not found" for exactly the users who
+followed the documented install, while working for anyone who happened to
+install system-wide — which is why it survives review so easily.
+
+Naming the absolute path sidesteps secure_path entirely.
+"""
+
+from __future__ import annotations
+
+import os
+import shutil
+import sys
+
+
+def self_path() -> str:
+    """Absolute path to this CLI's entry point.
+
+    Falls back to the bare name when the entry point cannot be resolved (an
+    unusual invocation such as `python -m`), because a slightly wrong hint is
+    better than a traceback while reporting an unrelated problem.
+    """
+    argv0 = sys.argv[0] or "bot-bottle"
+    resolved = shutil.which(argv0) or argv0
+    if not os.path.isabs(resolved):
+        if os.path.exists(resolved):
+            resolved = os.path.abspath(resolved)
+        else:
+            return "bot-bottle"
+    return resolved
+
+
+def sudo_command(*args: str) -> str:
+    """A copy-pasteable `sudo …` invocation of this CLI.
+
+    >>> sudo_command("backend", "setup", "--backend=firecracker")
+    'sudo /home/u/.local/bin/bot-bottle backend setup --backend=firecracker'
+    """
+    return " ".join(["sudo", self_path(), *args])