P2: shared sync check helper + PR discovery hardening

[brdv]

Summary

Apply P2 polish across sync/status discovery and documentation:

• remove duplicated first-open/default-root sync-check logic,
• harden PR discovery against silent limit truncation,
• align docs with new submit command surface.

Changelist

• src/stack.rs
  • Added first_open_branch_rooted_on_default helper to centralize first-open/default-root detection.
  • Added unit tests for helper behavior.
• src/cli.rs
  • Replaced duplicated first-open/default-root logic in both status and sync planning with shared helper.
• src/github.rs
  • Increased PR list discovery limit from 500 to 1000.
  • Added explicit guard error when returned list hits limit, preventing silent truncation assumptions.
  • Added unit test for limit guard.
• README.md
  • Added submit to command surface.
• USAGE.md
  • Added submit usage section and behavior details.

Validation

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