Adopt shared errorMessage() in agents.ts and templates.ts

[selfcontained]

Summary

• Replaced 5 inline error instanceof Error ? error.message : "..." patterns in agents.ts with the shared errorMessage() helper from shared/lib/error-message.ts
• Replaced 2 remaining inline patterns in templates.ts (which already imported the helper but wasn't using it everywhere)
• This is a continuation of the tech-debt effort from PR Extract duplicate constraint-error handler in personalities.ts #602 which adopted errorMessage() in personalities.ts

Why this is tech debt

The shared errorMessage() helper was introduced specifically to eliminate this boilerplate. These 7 remaining inline instances were the last holdouts — using them inconsistently defeats the purpose of the shared helper and makes future changes (e.g., adding logging or changing the fallback behavior) require touching every file instead of one.

What's next

Next tech-debt run will tackle the top backlog item: monitoring the ide-settings.ts / agent-type-settings.ts pattern similarity.

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