check:generated fails on dev: HttpApiError.BadRequest produces invalid TS identifier #246

What happened

After fixing #245 (ScheduleValidationError missing its literal discriminator), bun run check:generated gets further but still fails, now with:

error: Failed to format types.ts: SyntaxError: '=' expected. (11:19)
   9 | export const isInvalidRequestError = (value: unknown): value is InvalidRequestError => typeof value === "object" && value !== null && "_tag" in value && value["_tag"] === "InvalidRequestError"
  10 |
> 11 | export type effect/HttpApiError/BadRequest = { readonly "_tag": "BadRequest";  }
     |                   ^
  12 | export const iseffect/HttpApiError/BadRequest = (value: unknown): value is effect/HttpApiError/BadRequest => typeof value === "object" && value !== null && "_tag" in value && value["_tag"] === "BadRequest"

Reproduced locally on a clean dev checkout (with #245's fix applied on top, using [email protected] as pinned in root package.json) — this is not caused by any PR, it's the next layer of a chain of masked failures (formatMinutes locale → ScheduleValidationError discriminator → this).

Root cause

packages/protocol/src/groups/system.ts's system.update endpoint declares error: [HttpApiError.BadRequest], using effect's built-in HttpApiError.BadRequest class directly instead of a repo-defined error class. packages/httpapi-codegen/src/index.ts's declaredErrorFields() resolves this class's identifier via SchemaAST.resolveIdentifier(schema.ast), which for this built-in class returns the internal module-qualified name effect/HttpApiError/BadRequest (slashes included) instead of falling back to the literal tag ("BadRequest"). That string is then emitted as a TypeScript type name, which is invalid syntax.

system.update (packages/server/src/handlers/system.ts:11, yield* new HttpApiError.BadRequest()) is the only place in the codebase using HttpApiError.BadRequest directly — every other endpoint uses a repo-defined error class with an explicit name/_tag literal (see #245).

Possible fixes (needs a decision, not attempting in this issue)

1. Narrow fix, follows existing precedent: replace HttpApiError.BadRequest in system.ts with a repo-defined error class (e.g. SystemUpdateError) carrying its own name literal, matching the ScheduleValidationError/ProjectCopyError pattern. Smallest change, no risk to httpapi-codegen itself.
2. General fix: make declaredErrorFields() in packages/httpapi-codegen/src/index.ts fall back to tag.literal when SchemaAST.resolveIdentifier() returns a string that isn't a valid TS identifier (e.g. contains /), instead of trusting it unconditionally. Fixes this for any future use of a built-in HttpApiError.* class, but touches shared codegen logic.

Filing as a separate issue rather than folding into #245 since it's a distinct root cause in a different file.