Add --owner flag for collaborator push and show username in whoami

[JannikSt]

• Add --owner/-o option to env push for collaborators
• Display username in whoami output

---

Note

Adds --owner/-o to env push, displays username in whoami, and improves sandbox run command handling with proper quoting and guidance.

• CLI:
  • Environment Push (packages/prime/src/prime_cli/commands/env.py):
    • Add --owner/-o flag and send owner_slug during resolve (preferred over --team).
  • Sandbox Run (packages/prime/src/prime_cli/commands/sandbox.py):
    • Use shlex.join to preserve argument quoting; enhance help/docs to use -- before command options.
  • Whoami (packages/prime/src/prime_cli/commands/whoami.py):
    • Fetch and display slug as "Username" in the output table.

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