feat: prepare managed controls for skills and marketplaces

[viyatb-oai]

Why

The enforcement PRs need a shared in-memory representation for managed skill-source and plugin-marketplace policy before the public requirements.toml surface is activated. Keeping that plumbing separate lets the later activation PR land only after the behavior exists.

This PR is intentionally internal-only: it does not yet make any new managed config keys parseable from requirements.toml.

What changed

• Added the normalized ConfigRequirements fields for skill-source and plugin-marketplace policy.
• Added shared requirement value types for skill sources and plugin marketplace allowlists.
• Exported those shared types for the enforcement layers that follow in the stack.

Example config.toml

Once the later activation PR is present, an approved marketplace can still appear in user config.toml like this:

[features]
plugins = true

[marketplaces.approved-marketplace]
source_type = "git"
source = "https://github.com/example/approved-marketplace.git"

There is intentionally no user-writable allowed_sources key in config.toml; skill-source restrictions are enforced only from managed requirements.

Verification

• This PR only adds the internal model consumed by the enforcement PRs above it.

Stack

--> #21462: internal managed artifact requirement plumbing

• feat: restrict which skill sources Codex can load #21457: managed skill enforcement
• feat: restrict which plugin marketplaces Codex can use #21458: core plugin allowlist enforcement
• feat: enforce marketplace restrictions at plugin entry points #21459: plugin entrypoint enforcement
• feat: let admins configure managed skill and marketplace controls #21413: managed artifact requirements activation