[ENG-3771] projects-cli-1: default project UX

[mrmoxon]

Summary

• stack on [ENG-3771] projects-cli-0: explicit projects CLI #731 and add the behavior-changing project defaults
• make prime lab setup create, select, or bind an active Lab project unless --no-project is passed
• have train/eval creation paths use the active project by default, while prime eval push --eval-id avoids implicit reassignment
• update README guidance for the default-project workflow

Review notes

This PR intentionally contains only the default UX layer on top of #731. The explicit projects CLI, API clients, assignment commands, and explicit --project plumbing live in #731.

Validation

• uv run pytest packages/prime/tests/test_rl_config.py packages/prime/tests/test_lab_setup.py packages/prime/tests/test_eval_push.py packages/prime/tests/test_hosted_eval.py packages/prime/tests/test_projects_cli.py packages/prime/tests/test_rl_api.py packages/prime/tests/test_deployments.py packages/prime-evals/tests/test_evals.py -q
• uv run ruff check packages/prime/src/prime_cli/commands/rl.py packages/prime/src/prime_cli/lab_setup.py packages/prime/src/prime_cli/commands/evals.py packages/prime/src/prime_cli/utils/eval_push.py packages/prime/src/prime_cli/verifiers_bridge.py packages/prime/tests/test_lab_setup.py packages/prime/tests/test_eval_push.py packages/prime/tests/test_hosted_eval.py

---

Note

Medium Risk
Behavior change: new runs/evals may attach to a workspace project without explicit --project; eval push with --eval-id is guarded, but mis-scoped active projects could group artifacts unexpectedly.

Overview
Lab workspaces now default to an active project when you run prime lab setup (if logged in): it creates or reuses a project from the folder name, or you can bind with --project, name with --project-name, or skip with --no-project. prime project clear records a cleared state in workspace context instead of deleting the file, so setup won’t silently re-attach a project until you opt back in.

Train, eval run, and eval push resolve --project / --no-project against the active workspace project by default (resolve_project_id with use_active_project). Hosted training always uses the active project; local eval run only when uploads aren’t skipped; eval push uses it for new pushes but not when --eval-id is set (avoids implicit reassignment on updates). Help text and READMEs document the new flow and --no-project on train/eval push.

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