Remove optional provider configuration discovery

[SaladDay]

Remove optional provider-configuration discovery. Startup configuration returns its original four-field snapshot and rejects query parameters; the client opt-in, public schema and unused Runtime provider descriptors are removed. Internal provider validation, Agent defaults and frozen Session execution-configuration reads remain unchanged.

The startup route remains an explicit Core extension. Official documentation and read-only API probes confirmed that OpenAI does not expose this Core route; no replacement discovery API or unrelated protocol changes are introduced. Old Runtime heartbeats with the retired field remain accepted.

Validation: regenerated OpenAPI, identical sqlc output, affected and broad Go/PostgreSQL checks, standalone builds, all 394 client tests, 715 Web unit tests, TypeScript, Claude SDK packaging, MiniMax and Rust checks. Independent blind review found no actionable issues. Browser acceptance on isolated ports passed 123 of 127 cases; the four failures match clean-baseline Dashboard failures.

The full gate is not green: clean baseline 3fb5f87 independently reproduces the KVM-dependent installer failure and cascading fixture-directory error, plus four existing Dashboard browser failures. Initial local toolchain, test-environment and occupied-port issues were corrected without source changes. Production services are unchanged.