Generate config schema from Effect Schema

[kitlangton]

Summary

• Generate the public config.json schema directly from Config.Info with Schema.toJsonSchemaDocument
• Normalize Effect's optional-field JSON Schema output so editor validation does not allow null values that runtime config parsing rejects
• Restore the external models.dev $ref for model fields, preserving editor model validation/autocomplete

Compatibility

• Runtime config parsing is unchanged; this only changes the build-time public JSON Schema generator
• Generated tui.json is byte-for-byte unchanged
• Generated config.json now uses standard $defs/$ref instead of inlining repeated schemas, but preserves JSONC flags and model refs
• Verified generated config.json has no anyOf branches allowing null

Verification

• bun run script/schema.ts /tmp/config.json /tmp/tui.json
• diff -u <before-tui.json> <after-tui.json> produced no diff
• Generated config schema invariants: no nullable optional branches, restored models.dev refs, allowComments, allowTrailingCommas
• bun typecheck
• bunx prettier --check script/schema.ts
• bunx oxlint packages/opencode/script/schema.ts