[Feature] Support discovery, installation, and management of commands

#### Problem Statement / Context
Currently, `opencode-market` is fantastic for managing and installing **agents** and **skills** from GitHub marketplaces into global (`~/.agents/`), project-local (`./.agents/`), or OpenCode folders (`./.opencode/`). However, it doesn't currently support standalone **commands**.

To maintain consistency across all extension types in OpenCode ecosystem workflows, it would be extremely helpful if marketplaces could also declare, distribute, and install commands.

#### Proposed Solution

1. **Update `marketplace.json` Schema:**
Allow marketplaces to declare a `commands` array alongside `agents` and `skills`.

2. **Add Installation Target Paths:**
Map command installation paths mirroring the existing structure:
* **Global (default):** `~/.agents/commands/`
* **Project-local (`--local`):** `./.agents/commands/`
* **OpenCode folder (`--opencode`):** `./.opencode/commands/`

3. **Update CLI & `list` Command:**
Ensure the `install`, `update`, and `list` commands correctly parse, fetch, and display commands alongside existing plugin types.

Start by tracing how the existing agents and skills are represented in marketplace.json and handled by the install, update, and list commands. Verify command discovery, display, and installation for the global, --local, and --opencode targets; done means commands work consistently alongside the existing plugin types.

Written by the indexing model from the issue text.