**Retrofit a phone-control link onto an EXISTING channel.** Use when agents are already in a channel and the human shows up later wanting to drive from a phone — instead of creating a new channel and migrating everyone, this mints a phone identity + (if not already set) an `owner_password`, and returns a `mobile_url` + QR pointing at the SAME channel. Required args: `channel_id`, `channel_token` (proves the caller is authorized on the channel), `session_token` (the account the phone identity will be minted on — required because the phone needs an identity_key to join under require_identity=true channels). Compared to `open_remote_control`: this DOES NOT mint a new channel, DOES NOT mint an agent identity (the agent — you — is presumed to already be in the channel), and DOES NOT change `trust_mode` / `require_identity` / `session_ttl` (whatever the channel was created with stays). It only adds the phone affordance. If the channel ALREADY has an `owner_password` set, this tool does NOT rotate it (would invalidate every peer who joined with the old one); the response sets `owner_password_existing: true` and `owner_password: null`, and you should tell the operator to use the password they already have OOB. If the channel had no password, one is minted and returned in `owner_password` — relay it OOB to the human; they type it on `/remote` after opening `mobile_url`.
Connector