Add stck submit command

[brdv]

Summary

Add stck submit to create a PR for the current branch when one is missing.

This closes the dogfooding gap where users had to run manual gh pr create ... after adding commits.

Changelist

• src/cli.rs
  • Added submit subcommand: stck submit [--base <branch>].
  • Behavior:
    • rejects default branch submission,
    • ensures upstream exists (pushes -u when missing),
    • no-ops if PR already exists,
    • creates PR for current branch with optional base override.
  • Updated stck new no-commit guidance to recommend stck submit --base <parent>.
• tests/cli_skeleton.rs
  • Help output now asserts submit command is listed.
  • Updated no-commit guidance expectations.
  • Added tests:
    • submit_creates_pr_with_base_override
    • submit_defaults_base_to_default_branch
    • submit_noops_when_pr_exists
    • submit_rejects_default_branch

Validation

• cargo fmt --all
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features