Docs/Skills updates from feedback & LLM benchmark run

[bradleyshep]

Description of Changes

Fixes stale and missing API guidance in the server skills and core-concepts docs, found by triaging the periodic LLM benchmark (C# and TypeScript both scored 0 passes while all golden answers publish) and cross-referenced against external developer feedback (SIMPLI FEEDBACK.md).

Key corrections:

• Table accessor casing. typescript-server/SKILL.md and tables.md claimed ctx.db accessors are the camelCase form of the table name. Since Case conversion #4263 (2026-02-20) accessors are the literal key passed to schema({...}), verbatim. This was the single biggest TS benchmark failure and matches SIMPLI's casing complaint.
• Canonical-first skills. Both server skills now open with a short complete module and list the full set of imports / column modifiers / attributes. Targets the ~85% of benchmark failures where models emitted the old decorator/attribute API.
• C# gaps. Fully-qualified [SpacetimeDB.Index.BTree] (avoids System.Index ambiguity, CS0104); tagged-enum declaration + variant construction (avoids CS8910); scheduled table and reducer kept in one class so nameof(...) resolves (CS0103).
• Wrong type name in TS docs. ReducerContext does not exist in TypeScript; models generated it 55x in the benchmark. Skill now names the real type: ReducerCtx<InferSchema<typeof spacetimedb>>.
• Rust syntax error. Entity { ..existing, name } does not parse; ..existing must come last.
• Prefix-scan fix. A multi-column prefix scan passes the bare leading value, not a shorter array.
• Token/identity guidance. Corrected the auth notes in three quickstarts and the Unity/Unreal skills: server-issued (not "self-issued") identity, persisting the token keeps identity on reconnect, and the token is non-expiring so production should use an OIDC provider such as SpacetimeAuth. Consistent with the auth doc and FAQ.
• Smaller doc touch-ups from SIMPLI feedback: client-binding casing note, reducer-arg array support, RLS how-to.

Net: the two server skills are ~74 lines smaller than before.

API and ABI breaking changes

None. Docs and skills only.

Expected complexity level and risk

1. Documentation and skill markdown, no code paths touched. The skills ship in the CLI binary (crates/cli/build.rs) and sync to docs/static/.well-known/, so the only interaction is that these files are embedded at build time.

Testing

• Compiled a module exercising the full TypeScript skill surface (builders, modifiers, custom types, indexes, scheduled tables, views, ReducerCtx helpers) against crates/bindings-typescript at 2.7. The camelCase accessor spelling reproduces the exact CI error.
• Traced every C# claim to crates/bindings-csharp source or to CI-validated goldens.
• Reviewer: confirm cd docs && pnpm generate-llms builds clean (regenerates llms.md, fails on broken links).
• Reviewer: sanity-check the next periodic benchmark run shows TS/C# publish failures dropping on the accessor-casing and CS0104/CS8910 tasks.