1. Added SessionSource::Custom(String) and --session-source.
2. Enforced plugin and skill products by session_source.
3. Applied the same filtering to curated background refresh.
Resubmit https://github.com/openai/codex/pull/15020 with correct
content.
1. Use requirement-resolved config.features as the plugin gate.
2. Guard plugin/list, plugin/read, and related flows behind that gate.
3. Skip bad marketplace.json files instead of failing the whole list.
4. Simplify plugin state and caching.
- 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.
