Add cowork host support (Claude Desktop agent mode)

## Request

Add `cowork` to the supported `--host` values in `setup` so gstack skills load in Claude Desktop's agent mode (Cowork), alongside the existing claude/codex/kiro/factory/opencode/openclaw/hermes/gbrain hosts.

## Why

Cowork is the local-agent runtime in the Claude desktop app. It runs the same Anthropic skills format (`SKILL.md` + frontmatter) but loads plugins from a different location and, critically, only via marketplace installs tracked in `rpm/manifest.json`. Today there's no documented path to install a custom skill bundle like gstack into Cowork, so users running both Claude Code and Cowork lose access to /qa, /review, /ship, /office-hours, etc. inside the desktop app.

## What I see locally

Cowork plugins live at:
`~/Library/Application Support/Claude/local-agent-mode-sessions///rpm/plugin_/`

Each plugin has:
- `.claude-plugin/plugin.json` (name, version, description, author)
- `skills//SKILL.md`
- A marketplace ID + entry in `rpm/manifest.json`

The `marketplaceId`/`marketplaceName` requirement is the blocker for side-loading. A `gstack setup --host cowork` would need to either:
1. Generate a `.claude-plugin/plugin.json` and install to a path Cowork scans outside `rpm/` (if such a path exists), or
2. Document the marketplace publishing path so gstack ships as an official Cowork plugin, or
3. Drop the skill bundle into a session-local `.claude/skills/` that Cowork honors (if it does — I haven't found docs).

## Environment

- gstack v1.26.4.0 (latest as of this filing)
- macOS 15.5
- Claude Desktop with Cowork enabled

Happy to test patches against my local Cowork install if you want a guinea pig.

Start with the existing `setup --host` handling and compare the Cowork paths and metadata described in `.claude-plugin/plugin.json`, `skills//SKILL.md`, and `rpm/manifest.json`. Determine which supported installation path is valid for Cowork; done means `gstack setup --host cowork` installs or documents a working Cowork-compatible skill bundle.

Written by the indexing model from the issue text.