fix: surface conflicting PRs

[shrirajpawar4]

Summary

• add a derived conflicting session status for open PRs with merge conflicts
• allow merge-conflict nudges even when the session is waiting_input
• add pr_conflicting notifications so conflicts reach the user even if the agent is paused
• show conflicting sessions in the frontend as Needs you / Conflicts instead of Ready to merge
• regenerate OpenAPI and frontend API schema for the new enum values

Notes

This intentionally keeps the change scoped to merge conflicts. The broader behavior where waiting_input can mask other PR-derived statuses is related to #290 and should be handled separately.

A new SQLite migration is included because notification type has a CHECK constraint and existing migrations should not be edited.

fix for #2173

Test

• go test ./internal/service/session ./internal/lifecycle ./internal/notify
• go test ./internal/httpd/...
• npm run frontend:typecheck
• cd frontend && npm exec vitest run src/renderer/types/workspace.test.ts src/renderer/components/NotificationCenter.test.tsx