copilot/gemini adapters fail when CLI is installed globally; cursor/opencode auth check too strict #5

copilot and gemini: `checkInstalled` fails for globally-installed CLIs

The `copilot` and `gemini` adapters use `resolvePackageBin` to locate `@github/copilot` and `@google/gemini-cli` respectively. This resolution walks the local `node_modules` chain from where spawn-agent itself is installed. When a user has the CLI installed globally (e.g. via `npm install -g` or a system package manager), the package is outside that chain and `checkInstalled` returns `false`, causing `resolve` to throw `AdapterNotFoundError`.

The other native-binary adapters (`cursor`, `opencode`, `droid`, `pi`) check for the binary using `runCommand(bin, ["--version"])` — a simple PATH lookup that works regardless of how the CLI was installed. The copilot and gemini adapters don't follow this pattern.

Observed:

• `copilot` or `gemini` is on `PATH` and works fine when invoked directly
• `SpawnAgent.connect("copilot", ...)` throws `AdapterNotFoundError: Adapter package not resolvable: @github/copilot`

Expected:

• `checkInstalled` uses binary detection (like cursor/opencode do), so a globally-installed CLI is found

cursor: wrong auth subcommand — `agent auth whoami` does not exist

The cursor adapter's `checkAuthenticated` and `resolve` both call `agent auth whoami`. The Cursor agent CLI has no `auth` subcommand. The listed subcommands are `login`, `logout`, `status|whoami`, `mcp`, `models`, etc. — there is no `auth`.

When `agent auth whoami` is run, the CLI treats `auth whoami` as a prompt to the agent process rather than a subcommand. Without credentials the process exits 1 immediately with an error; with credentials it starts an interactive session. Either way it never exits 0 with non-empty stdout on the auth-check path.

The correct command for checking authentication status is `agent whoami` (or equivalently `agent status`), which exits 0 and prints the logged-in account to stdout when authenticated.

Observed:

• `agent --help` lists `status|whoami` as the auth subcommand, not `auth whoami`
• `agent auth whoami` exits 1 (treated as a prompt, no auth)
• `agent whoami` exits 0 with non-empty stdout when authenticated
• `checkAuthenticated("cursor")` always returns `false` even when the CLI is ready to run

Expected:

• Auth check calls `agent whoami` (or `agent status`), which correctly reflects authentication state