feat: add FRANCIS_HOST to connect to a standalone Francis runtime

FRANCIS_HOST decides where the Francis actor runtime lives. When set to "embedded" (the default), Pocket ID starts the runtime inside its own process.

Any other value is the address, or a comma-separated list of addresses, of a standalone Francis runtime. Pocket ID then connects to it as a remote actor host and starts no embedded runtime.

Because when using a remote runtime, it's likewise not possible to enforce a single instance of Pocket ID is running at once, the env vars currently have the `EXPERIMENTAL_` prefix, are **undocumented**, and show a warning if used.

Notes:

- Connecting to a standalone runtime also needs FRANCIS_HOST_PSK or FRANCIS_HOST_JWT_FILE, and optionally (but recommended) FRANCIS_CA.
- When connecting to a remote runtime, exporting Pocket ID data does not include the actor state, which will need to be backed up and restored separately

AGENTS.md

@@ -27,6 +27,8 @@ End-to-end (needs Docker; **stop any local backend on `:1411` first** — see go
 
 ```sh
 cd tests/setup && docker compose up -d --build   # rebuild after ANY code change, or you test stale code
+# docker-compose-francis.yml runs the same suite against a standalone Francis runtime instead of the embedded one
+# It needs FRANCIS_VERSION in tests/setup/.env: echo "FRANCIS_VERSION=$(./francis-version.sh)" > .env
 cd ../.. && pnpm test                            # = playwright test in tests/
 ```