feat(submit): add --push-only flag

[boneskull]

Adds a new --push-only flag to gh stack submit that skips
Phase 3 (PR creation/update) and only performs cascade + push.

Useful when you want to push branches without creating or updating PRs.

• Errors if combined with --update-only or --web (both affect PRs)
• Flag is persisted in cascade state for conflict recovery via continue

[boneskull]

Stack

• main
  • feat(submit): add --push-only flag #25 - branch: submit-no-pr (this PR)
    • default-pr-title

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Managed by gh-stack

[Copilot]

Pull request overview

Adds a new --push-only flag to gh stack submit to allow running cascade + push while skipping the PR creation/update phase, including persistence across conflict recovery via gh stack continue.

Changes:

• Added --push-only flag to submit with validation against --update-only and --web.
• Persisted PushOnly in internal/state.CascadeState and wired it through cascade/continue flows.
• Added unit + e2e coverage for state persistence and --push-only behavior/flag conflicts.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
cmd/submit.go	Adds flag, validates conflicts, and skips PR phase when pushOnly is set.
cmd/cascade.go	Threads pushOnly through cascade state persistence on conflict.
cmd/continue.go	Restores pushOnly from persisted state when continuing a submit.
cmd/sync.go	Updates cascade helper call signature after adding pushOnly.
internal/state/state.go	Adds PushOnly to persisted cascade/submit state JSON.
internal/state/state_test.go	Unit test coverage for PushOnly state save/load.
e2e/submit_test.go	E2E coverage for --push-only output and conflicting flags.
README.md	Documents the new --push-only submit flag.

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