Add a versioned Provider preset manifest and curated catalog #89

Part of #71.

Outcome

Add a versioned, localizable CodexHub Provider preset manifest and a maintainer-approved initial catalog of approximately 6–10 tested Providers.

Scope

• Define stable preset IDs, display/localization keys, reviewed base URLs, upstream formats, tool protocol, conservative capabilities, supported auth methods, environment-variable names, documentation URLs, provenance, and tested dates.
• Convert existing bundled Provider definitions into the manifest.
• Propose the initial catalog from Providers that the Gateway and automated probes can exercise.
• Validate schema and provenance and keep a reviewed offline snapshot.
• Preserve the fully custom Provider path and Official OpenAI/ChatGPT subscription behavior.

Non-goals

• Credentials, OAuth, runtime Models.dev authority, or automatic mutation of user configuration.

Acceptance criteria

• Manifest schema and stable IDs are documented and validated.
• Initial 6–10 Provider catalog is explicitly approved by the maintainer.
• Every entry records source provenance and a tested date.
• Presets contain no secrets and use conservative capability claims.
• Existing bundled definitions migrate without changing user-owned configuration.
• Offline operation uses the reviewed bundled snapshot.

Required verification

• Schema/fixture validation tests.
• Provider registry and routing tests for every included preset.
• Localization/UI-contract checks for display metadata.