Warn when an agent plugin prompt file is missing

[TheGreatAxios]

Summary

• Emit one warning through the existing profile warning sink when an agent plugin prompt file cannot be read, naming the plugin id, agent id, attempted path, and first-line reason
• Load the profile without a role instead of silently swallowing the miss

Verification

• bun test src/plugins/agent-plugins.test.ts passes (new regression fails before the fix, passes after)
• bun run typecheck passes; lint and build pass via pre-commit and bun run check stages

Fixes CL-6724

[linear-code]

CL-6724