fix(workflow): fall back on malformed render details

[tt-a1i]

Summary

• validate workflow tool-result details before entering the custom card renderer
• fall back to Pi's plain text rendering for empty or structurally incomplete error details
• keep malformed results out of the Workflow spinner lifecycle
• add regression coverage for the exact parse-error crash path

Why

Pi represents some extension-tool execution failures with isError: true and details: {}. The Workflow renderer previously treated every truthy details value as WorkflowDetails, then called aggregateUsage(details.agents). A Workflow parse error therefore became an uncaught TypeError: agents is not iterable and terminated the TUI.

The fix checks the mandatory top-level Workflow render contract (runId, background, terminal/running status, finite startedAt, phases, and agents) before using the custom renderer. Invalid shapes preserve the original error text and do not start a spinner. Valid Workflow cards are unchanged.

Test plan

• node --test --experimental-strip-types extensions/workflows/rendering.test.ts
• bun run check
• bun run test — 868 Node tests and 30 Vitest tests passed
• git diff --check

Closes #101