fix(dashboard): initialize prompts at startup for System Prompt tab

[zbigniewsobiecki]

Summary

• Fix blank System Prompt tab in Agent Definition Editor by calling initPrompts() at dashboard startup
• Add proper error handling with Sentry capture and graceful exit on startup failure
• Match the async startup pattern used in src/router/index.ts

Root Cause

The dashboard API (src/dashboard.ts) was not calling initPrompts() at startup. When the frontend called trpc.prompts.getDefault, the API called getRawTemplate() which threw because prompts weren't initialized. The frontend had no error handling for this, resulting in a blank textarea.

Changes

• Add initPrompts() import and call in async startDashboard() function
• Add flush import from sentry for proper error handling
• Wrap server startup in named async function with .catch() error handler
• On startup failure: log error, capture to Sentry with level: 'fatal', flush events, exit with code 1

Test plan

• Lint passes (npm run lint)
• Typecheck passes (npm run typecheck)
• All 3557 unit tests pass (npm test)
• After deployment: Open dashboard → Agent Definitions → Select any agent → System Prompt tab shows content
• "Reset to Default" button populates editor with default template

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