fix(bootstrap): reuse the installed root domain instead of re-deriving it

detect_node_ip recomputed FELIS_ROOT_DOMAIN from scratch on every run and fell
back to <node-ip>.nip.io. Nothing read the domain back out of the felis.toml an
earlier run wrote, so it survived only as long as the operator kept passing the
same environment.

That made re-running the installer destructive on any install with a real
domain, and re-running it is not optional: it is the only way to move felis-api
to a newer release, which is what `felis update` points operators at. A bare
re-run rewrote root_domain, panel_hostname and admin_hostname to nip.io names
while ensure_panel_tls_cert returned early on the certificate it had already
written, leaving the console serving a cert for hostnames it no longer answered
to -- with no re-domain flow to recover through.

Precedence is now explicit FELIS_ROOT_DOMAIN, then the domain the last run
persisted, then the nip.io default. First installs are unaffected. Deliberate
re-domains still work, because there is no other route to one, but they now warn
that the write-once certificate is not reissued and that the proxy and login
config carry the old name too.

Secrets were never exposed to this: load_or_make_secrets has always sourced
secrets.env before generating anything. The domain was the one piece of install
identity with no read-back.

The channel is deliberately left alone. FELIS_VERSION_BOOTSTRAP is not persisted
either, but defaulting a re-run to the release channel installs a working build
rather than breaking one, so cmd/felis/update.go states that instead. Its
warning about the domain went with the bug and would now be false.

Verified against the shipped function text: the ladder holds for a fresh host,
a re-run with and without the variable set, a re-domain, and a felis.toml whose
root_domain is missing or empty. Reverting the one line reproduces the nip.io
overwrite.

cmd/felis/update.go

@@ -262,15 +262,14 @@ func renderApplyGuidance(res updater.Result, selected map[string]bool, force boo
 	// rebuilds the image and rolls the deployment from the SAME binary -- a run that looks
 	// like a successful update and leaves the version unchanged.
 	//
-	// The installer is the only thing that moves felis-api, but it is NOT an updater and
-	// must not be recommended as one without this warning. detect_node_ip re-derives
-	// FELIS_ROOT_DOMAIN on every run and defaults it to <node-ip>.nip.io -- nothing reads
-	// the domain back out of the felis.toml a previous run wrote. A bare re-run therefore
-	// rewrites root-domain/panel-hostname/admin-hostname to nip.io names while
-	// ensure_panel_tls_cert, which is write-once, keeps serving the old ones: the console
-	// stops matching its own certificate. There is no re-domain flow to recover with.
+	// The installer is the only thing that moves felis-api. It is safe to point at now
+	// that detect_node_ip reuses the installed root domain, so what is left to warn about
+	// is the channel: FELIS_VERSION_BOOTSTRAP is not persisted anywhere and defaults to
+	// release, so a bare re-run on a host tracking main quietly moves it onto releases.
+	// That is a channel change, not a broken install, which is why it is one clause and
+	// not a paragraph.
 	if offeredFelisAPI {
-		b.WriteString("\nfelis-api (panel, plugins) is the exception: setup re-images it from the felis binary\nalready on this host, so it cannot install a NEWER felis-api. Only re-running the\nbootstrap installer does that, and it is a full install run, not an update: give it the\nSAME environment as the original install, FELIS_ROOT_DOMAIN above all. It defaults to\n<node-ip>.nip.io, and a bare re-run re-domains this install while the write-once panel\ncertificate keeps the old hostnames.\n")
+		b.WriteString("\nfelis-api (panel, plugins) is the exception: setup re-images it from the felis binary\nalready on this host, so it cannot install a NEWER felis-api. Re-run the bootstrap\ninstaller for that -- it keeps this install's root domain. It does default to the\nrelease channel, so pass FELIS_VERSION_BOOTSTRAP=dev if this host tracks main.\n")
 	}
 	return b.String()
 }