Remove Shortcut references from UI to make AutoPR tool-agnostic

[Neokil]

Summary

• replace Shortcut-specific discovery modal copy with generic ticket wording
• add focused frontend coverage for discover modal copy and behavior
• keep backend discovery behavior unchanged

Testing

• cd web && npm run build
• cd web && npm run lint
• cd web && npm run typecheck
• cd web && npm test
• go test ./...
• golangci-lint run (currently fails on pre-existing generated-code and stale worktree issues outside this change)