feat(cli): add instance: false opt-out to effectCmd

[kitlangton]

Summary

For commands that don't read project state, the always-on `InstanceStore.provide` adds unnecessary work (`InstanceBootstrap` = config + plugin init + LSP/File/Snapshot/Vcs/Format/ShareNext init forks) and emits a `server.instance.disposed` IPC event the TUI re-bootstraps on.

Add `instance: false` to opt out: handler runs directly under `AppRuntime` with no instance load and no auto-dispose. Default stays `true` (no breaking change for existing converted commands).

```ts
effectCmd({
command: "serve",
instance: false, // ← skip InstanceStore.provide
handler: Effect.fn(...)(...)
})
```

First user: serve.ts

Converted as the demo. Server loads instances per-request via the `x-opencode-directory` header, so it never needed an ambient one — keeping the legacy "no instance load" semantics while still gaining `effectCmd`'s shape.

Future candidates (not in this PR)

`web`, `account` (login/logout/switch/orgs/open/console), `db` (path/query/migrate), `upgrade`, `uninstall` — all currently still on raw `cmd()` because the always-load behavior would add startup work they don't need.

Behavioral notes

• `models` was considered but cannot opt out: `Provider.list` reads `InstanceState` (per-instance config + plugins + auth), so it genuinely needs the project instance.
• For `serve`: identical observable behavior (no instance load before; no instance load now). Just the same shape via `effectCmd`.

Test plan

• `bun run typecheck`
• `bun run test test/cli/` — 137 pass
• Smoke: `serve --port 0` starts and listens