feat(step-failure-diagnostics): preserve typed session error diagnostics

[dagoaie]

Why

A session.error event carries a typed classification from the OpenCode SDK (name, data.statusCode, data.isRetryable, data.providerID), but describeSessionActivity flattens it to a message string and watchSession rethrows it as a plain Error. The "step failed — waiting for your decision" gate, the attempt log and metadata.json cannot tell a retryable 429 from an expired credential or a truncated output. As a side effect, a MessageAbortedError delivered through session.error was never recognised as a cancellation.

Summary

• add SessionErrorSignal { name, message, statusCode?, isRetryable?, providerID? } built from the event; formatEventError returns the exact same string, so activity lines do not change
• wrap session failures in SessionError at the attempt boundary (event-delivered and message-level info.error alike); name is pinned to "SessionError" and the SDK name lives in signal.name, so provider data can never match a runner sentinel such as isUserAbortError
• keep Claude Code failures (plain strings) unclassified; recognise an event-delivered MessageAbortedError as SessionAbortedError
• disclose the classification in the failure gate and log.warn: <message> (HTTP 429, retryable); unchanged when there is no status
• persist it as PhaseMetadata.error on failed phases via phaseFailed(name, detail?, failure?) / phaseEnded(name, status, failure?); an operator abort at the gate keeps the attempt failure as cause, so metadata records what actually failed; cancelling the message itself records nothing
• no automatic retry, no gate or exit-code change, no CLI/config/control-protocol change; run-status.ts and the TUI are untouched

The change was authored as OpenSpec change typed-session-errors (new capability step-failure-diagnostics) and landed with convoy close, so this PR includes the archived change and the generated openspec/specs/step-failure-diagnostics/spec.md. Happy to drop the archive/spec files if you prefer to land those yourself.

Verification

• bun run typecheck
• bun test (3207 pass)
• bun run test:coverage (92.6 %)
• openspec validate typed-session-errors --strict
• headless smoke with an invalid Anthropic key (--no-tui): the gate shows … (HTTP 401, not retryable) and metadata.json carries error. Note: an invalid credential reaches Convoy as APIError 401 isRetryable: false, not ProviderAuthError; the classification is carried as the SDK emits it.

🤖 Generated with Claude Code

https://claude.ai/code/session_018yC7LkEoDKjy61RzyGy8W2