Support pull requests in prompt sequences

[cosentinode]

Summary

• allow deferred open pr steps in prompt sequences while preserving the confirmation preview
• re-plan PR state only after commit and push predecessors succeed, stopping on planning, setup, auth, or execution failures
• cover the full commit/push/PR flow and short-circuit boundaries with fake CLI tests and smoke documentation

Verification

• cargo fmt --all --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo test --locked --workspace
• fake-CLI smoke tests for successful creation, commit failure, push failure, missing gh, auth failure, and sanitized PR audits

Fixes #47

[cosentinode]

Zero concerns found. Reviewed the complete diff against issue #47, current CI/checks, sequence planning and short-circuit behavior, the existing single-step pull-request validation/execution paths, audit sanitization, and surrounding tests. Local verification: cargo fmt --all --check, cargo test --locked --workspace, and git diff --check all pass.