- Add shared Product support to marketplace plugin policy and skill
policy (no enforced yet).
- Move marketplace installation/authentication under policy and model it
as MarketplacePluginPolicy.
- Rename plugin/marketplace local manifest types to separate raw serde
shapes from resolved in-memory models.
make plugins' `defaultPrompt` an array, but keep backcompat for strings.
the array is limited by app-server to 3 entries of up to 128 chars
(drops extra entries, `None`s-out ones that are too long) without
erroring if those invariants are violating.
added tests, tested locally.