[UX] ACP session errors should include actionable remediation hints

[yurtzy]

When an ACP session fails (e.g., acpx run), the error output is often terse: "session ended" or "connection closed" without explaining why or how to fix it.

Example pain point: A session exits with code 1 but logs show no clear cause — was it a model error, network timeout, auth failure, or plugin crash?

Proposed improvement: Include in error summaries:

• The exact failure point (which tool/agent threw)
• Suggested next steps based on common causes:
  • If model API returns 429/500 → "check provider rate limits or switch model"
  • If session times out → "increase --session-ttl or check network"
  • If plugin error → "verify plugin is installed and compatible"
• Reference to relevant config docs

Impact: Reduces debugging time for headless ACP users who lack interactive context.

This is a pure UX enhancement with no breaking changes.