Wire structured error propagation through UnitResult

[igouss]

Problem

UnitResult has status: "error" but nothing ever produces it — dead code. All error context (provider errors, timeouts, idle watchdog kills) is discarded at the resolve boundary:

• resolveAgentEnd(event) always produces { status: "completed", event }
• resolveAgentEndCancelled() always produces { status: "cancelled" } with zero context

The journal's error classification relies on regex-matching message content — fragile and unreliable across providers.

Solution

Add typed ErrorContext to UnitResult and wire it through the 4 call sites that discard error information. Replace regex heuristics in stuck detection and journal emit with structured field access.

Implementation

PR: gsd-build#2612

Files changed

• auto/types.ts — ErrorContext interface + field on UnitResult
• auto/resolve.ts — resolveAgentEndCancelled(errorContext?)
• auto-timers.ts — idle/hard timeout error context
• auto/run-unit.ts — session error/timeout context
• auto/phases.ts — structured classification + journal enrichment
• 3 test files updated