feat: add project resource and project-scoped defaults

[edouardb]

Summary

• add a new projects top-level resource with create/get/list/update/delete/describe/switch
• add a root --project/-p context, project name completion, and project-aware ID mappers
• scope apps and other project-aware resource list/create flows to the active project, with koyeb apps list --all-projects as an override

Notes

• TODO: the pinned API client in this repo exposes ProjectsApi and project_id request fields, but it does not expose Organization.default_project
• until that field is available in the generated client, projects switch persists the selected project in local CLI config and organizations switch clears it to avoid cross-org leakage

Verification

• go test ./...