make `AVAILABLE` the default plugin installPolicy when unset in
`marketplace.json`. similarly, make `ON_INSTALL` the default authPolicy.
this means, when unset, plugins are available to be installed (but not
auto-installed), and the contained connectors will be authed at
install-time.
updated tests.
wire plugin marketplace metadata through app-server endpoints:
- `plugin/list` has `installPolicy` and `authPolicy`
- `plugin/install` has plugin-level `authPolicy`
`plugin/install` also now enforces `NOT_AVAILABLE` `installPolicy` when
installing.
added tests.
1. Add a synced curated plugin marketplace and include it in marketplace
discovery.
2. Expose optional plugin.json interface metadata in plugin/list
3. Tighten plugin and marketplace path handling using validated absolute
paths.
4. Let manifests override skill, MCP, and app config paths.
5. Restrict plugin enablement/config loading to the user config layer so
plugin enablement is at global level
