feat: surface gate detail in the workflow run/resume --json payload

[doquanghuy]

Description

Reference implementation for #2964 — for discussion, direction welcome.

When a run pauses at a gate, the --json outcome now carries a gate block (step_id / message / options / choice) so orchestrators can detect "human review needed" and present the options without parsing the human-facing stream. Two small pieces:

1. The engine records each step's type in the run state's step results (one added line in step_data — previously the type was not recoverable from state).
2. _workflow_run_payload adds the gate block via a _gate_outcome helper when the run's current step is a gate. choice populates when the outcome ends at the gate with a decision recorded (e.g. an interactive rejection with on_reject: abort → a failed payload carrying "choice": "reject"; an on_reject: retry pause likewise). A mid-flow approval proceeds past the gate, so the block clears — by design. Non-gate runs and runs that end elsewhere are unchanged — no gate key, payload byte-identical to today.

The issue lists alternatives (a generic paused_step block; a dedicated status value) — happy to rework toward either.

Testing

• Ran existing tests with uv sync && uv run pytest — full suite 3727 passed
• Two new CLI-level tests (TestWorkflowRunGateOutcomeJson): a gate pause carries the exact block (CliRunner stdin is non-TTY, so the gate pauses); a completed run has no gate key — the gate-pause test is red against current main, green with the change (verified both directions)
• uvx ruff check src/ — clean
• Tested locally with uv run specify --help
• Tested with a sample project (covered by the CLI-level tests, which drive a real gate workflow through workflow run --json)

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

Code, tests, and this description were authored with AI assistance (Claude); verified by running the repo's test suite and ruff locally in both red and green directions.

[doquanghuy]

@mnriem when you have a moment, would appreciate your thoughts on the direction here — the issue lists the alternatives considered, and I'm happy to rework toward whichever shape fits Spec Kit best.