cli: no way to set a server-side session directory with --server

Summary

When a client connects to an OpenCode server on another host (opencode --server <url>), there is no CLI way to start a session in a directory that exists on the server. All directory inputs are resolved on the client, so cross-host workflows need a raw HTTP API call to create the session first, then -s to open it.

Environment

• opencode version: 2.2.1
• OS: Linux 7.1.8-200.fc44.x86_64 (client); headless Linux server (same version, reached via SSH port-forward)
• Terminal: TERM_PROGRAM=ghostty, TERM=xterm-ghostty, COLORTERM=truecolor
• Shell: zsh
• Install/channel: latest
• Active plugins: opencode-secrets, rtk, sound-notify, @tarquinen/opencode-dcp

Reproduction

1. On the server, start the opencode service and note a project path that exists only there, e.g. /home/alice/myproject.
2. On the client (different home dir, project not checked out), forward the port (ssh -L 49380:127.0.0.1:49380 server) and run opencode --server http://127.0.0.1:49380 /home/alice/myproject.
3. The client resolves the positional directory locally → fails with ENOENT before contacting the server.
4. Launching instead from a local cwd (e.g. ~/work): the TUI starts but the session lands at the server's current location, not the project; CLI subcommands (opencode session list) error with Location not found: /home/bob/work because the client cwd doesn't exist server-side.
5. The only working route is the HTTP API: POST /api/session with {"location":{"directory":"/home/alice/myproject"}}, then opencode --server <url> -s <session-id>.

Expected Behavior

A CLI-level way to express "start a new session at this directory on the server" — e.g. a --remote-dir/--location flag, resolving the positional path server-side when --server is set, or a setter for the server's current location (/api/location is GET-only today).

Actual Behavior

The positional <directory> argument and cwd are client-side only. A path that exists only on the server fails on the client (ENOENT); a path that exists only on the client fails on the server (Location not found). When client and server have different home directories, no single path can satisfy both.

Additional Context

Workaround wrapper (what we ship today):

sid=$(curl -u "opencode:$pw" -X POST "$url/api/session" \
  -H 'Content-Type: application/json' \
  -d '{"location":{"directory":"/home/alice/myproject"}}' \
  | jq -r '.data.id')
OPENCODE_PASSWORD="$pw" opencode --server "$url" -s "$sid"

Same cross-host setup (TUI against a remote opencode service over an SSH forward) as #62063.