feat!: redesign TaggedError — flat fields, sealed .withMessage()

[braden-w]

Summary

Redesigns createTaggedError from first principles based on analysis of 321 error call sites across the Epicenter codebase.

• Flat fields replace nested context/cause — error.status instead of error.context.status. .withContext()/.withCause() replaced by .withFields<T>()
• Sealed .withMessage() — two mutually exclusive modes: without it, message is required at the call site; with it, the template owns the message entirely (message not in the factory input type)
• Factories available immediately — .withMessage() is optional, not a required terminal step. The builder returns usable factories at every stage
• NoReservedKeys simplified — only name is reserved; message is either a built-in input or absent depending on .withMessage() usage

Before

const { ResponseError } = createTaggedError('ResponseError')
  .withContext<{ status: number }>()
  .withMessage(({ context }) => `HTTP ${context.status}`);
ResponseError({ context: { status: 404 } });             // template message
ResponseError({ context: { status: 404 }, message: 'x' }); // override

After

// With sealed .withMessage() — template owns message
const { ResponseError } = createTaggedError('ResponseError')
  .withFields<{ status: number }>()
  .withMessage(({ status }) => `HTTP ${status}`);
ResponseError({ status: 404 });  // message: "HTTP 404"

// Without .withMessage() — message required at call site
const { SimpleError } = createTaggedError('SimpleError');
SimpleError({ message: 'Something went wrong' });

Why seal the message?

Analysis showed no error type uses .withMessage() as a default that some call sites use and others override — it's always all-or-nothing. The override was an escape hatch masking two design problems: (a) the template needs better fields, or (b) the error should be a different type. 59% of call sites use sealed .withMessage(), 41% skip it and require call-site message.

Test plan

• 44 unit tests pass (bun test src/error/createTaggedError.test.ts)
• Full test suite passes (57 pass, 1 pre-existing failure from missing arktype dep)
• Build succeeds (bun run build)
• Specs, docs, README, CHANGELOG, ERROR_HANDLING_GUIDE all updated
• Verify no downstream consumers rely on message override with .withMessage()