refactor(api): trim schema generation and unify text types

[SaladDay]

The official-schema migration retained unused request projections, duplicate text configuration types, redundant field-order overrides, and repeated Go generation. Remove that maintenance overhead while preserving the public contract and stored encodings.

• Remove seven unused Go projections and consolidate six text configuration types into three. Saved Agents and Sessions share the same resolved text types and verbosity validation.
• Drop binding overrides that match the official source and the unused field-name override option. Generate Go once before Swag, then generate the public document from the extension definitions.
• Update the contract guide in English and Chinese. The public and Runtime schemas are byte-for-byte unchanged; the Core schema only merges two equivalent definitions.

Net change: 216 fewer lines. Validation: generator freshness and nine generator tests; five official-schema tests; public contract, Core API, item, Session and OpenAPI splitter tests; all Core and Go client packages compile; three translation checks. Independent review found no substantiated in-scope issues and independently passed generation and API tests. Official-client CI has passed; remaining CI jobs are still running with no failures observed.