fix(agent): reject invalid output modes

[JerryNee]

Summary

• validate testsprite agent ... --output <mode> against the supported json|text modes
• return the existing local VALIDATION_ERROR shape instead of silently treating unsupported modes as text
• add command-wiring coverage for agent list

Before

testsprite agent list --output yaml exited 0 and printed text output.

After

testsprite agent list --output yaml exits 5 with:

Error: Invalid request.
Flag `--output` is invalid: must be one of: json, text.

Verification

• npm test -- src/commands/agent.test.ts
• npm run lint
• npm run typecheck
• npm test
• npm run build
• node dist/index.js agent list --output yaml