Surface real runtime errors instead of "Unknown error"

[sean808080]

Summary

When Claude returns useful error text on stdout, the current forwarding paths can drop it and show Unknown error instead.

This change improves error extraction for daemon forwarding, Telegram, and Discord by:

• preferring stderr when present
• extracting structured JSON error messages when stdout is a Claude error envelope
• falling back to plain-text stdout when that contains the real message

Why

This preserves actionable runtime failures like Prompt is too long and auth errors, instead of collapsing them into a generic placeholder.

Validation

• bun x tsc --noEmit

[sean808080]

Closing in favour of #58 which takes a better approach: rather than patching a single call site, PR #58 introduces a shared src/messaging.ts utility module with an extractErrorDetail() function that handles the JSON stdout parsing in one place, then updates all three chat bridges (start.ts, telegram.ts, discord.ts) to use it. Same bug fixed, no duplication.