Polish sync/new guidance and usage docs

[brdv]

Summary

Polish user guidance and docs for the newly added default-branch new flow and sync --reset recovery flow.

This keeps command behavior and user-facing instructions aligned, and makes recovery paths clearer without changing core sync/push mechanics.

Changelist

• src/cli.rs
  • Improved sync state messaging:
    • resume message now points to stck sync --reset for explicit recompute
    • missing-state message for --continue now instructs to run stck sync to compute a plan
• USAGE.md
  • Updated new section to document both:
    • stacked branch flow
    • starting from default branch
  • Documented explicit recovery options:
    • stck sync --continue
    • stck sync --reset
  • Updated no-commit behavior text to reflect printed follow-up command.
• README.md
  • Added brief note that stck new <branch> works from default branch and stacked branches.
• tests/cli_skeleton.rs
  • Updated assertion for revised sync --continue no-state message.

Validation

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