feat(cli): add openshell gateway list subcommand

[maxdubrinsky]

Problem Statement

diagnose_port_conflict() in crates/openshell-bootstrap/src/errors.rs:248
prints this when openshell gateway start hits a port conflict:

Or stop other openshell gateways:
  openshell gateway list && openshell gateway destroy --name <name>

But openshell gateway list doesn't exist. The closest thing today is
running openshell gateway select with no argument, which falls through
to a listing in non-interactive mode (crates/openshell-cli/src/run.rs:919).

Proposed Design

Add List to GatewayCommands that calls the existing
run::gateway_list(&cli.gateway). Starting with no parameters to keep the
change small; we probably want to add the ones from sandbox list
(--limit, --offset, --ids, --names, --selector) sometime soon.

Alternatives Considered

Update the hint in errors.rs:248 to point at gateway select instead.
Simple enough if we want to treat gateways and sandboxes as different —
there is no sandbox select, after all. But we already have sandbox list,
so adding gateway list matches and feels right.

Agent Investigation

• Dead reference: crates/openshell-bootstrap/src/errors.rs:248 — only
  occurrence of openshell gateway list in the tree.
• run::gateway_list() already renders the table at
  crates/openshell-cli/src/run.rs:1349; only invoked from the
  gateway_select_with helper at run.rs:919.
• Sibling variants (Logout, Select, Info) at main.rs:1009-1033
  establish the shape to mirror.
• No changes to run.rs, no protobuf or server changes.