Split Integration Tests by Command

[brdv]

Summary

Split the monolithic integration suite into command-focused test files and move shared test support into a reusable harness module.

This keeps the suite easier to navigate as stck grows and removes the remaining shared fixed log filenames under std::env::temp_dir(). The behavior coverage stays the same, but each workflow now lives in a much more reviewable place.

Changelist

• tests/harness/mod.rs: Extract the shared stubbed git / gh tooling harness and add small helpers for per-test temp paths and resumed-command setup.
• tests/cli.rs, tests/new.rs, tests/submit.rs, tests/status.rs, tests/sync.rs, tests/push.rs: Split the old integration coverage by command/workflow.
• tests/cli_skeleton.rs: Remove the old monolithic integration file.

Checklist

• Changes are minimal and focused for this milestone
• No breaking CLI changes unless explicitly intended
• Errors are user-facing, actionable, and prefixed with error:
• Added/updated tests for behavior changes
• Updated docs/plan if behavior or scope changed