feat(workflows): opt-in output_format: json exposes parsed shell stdout as output.data

[doquanghuy]

Description

Reference implementation for #2962 — for discussion, direction welcome.

Adds an opt-in output_format: json to shell steps: when declared, stdout is parsed and exposed under output.data (the raw stdout/stderr/exit_code keys are unchanged, so there is no merge/clobber ambiguity), letting later steps consume typed values — e.g. a fan-out's items: "{{ steps.emit.output.data.items }}". A parse failure fails the step with a clear error (declaring the format is a contract; silence would hide wiring bugs). Without the key, behavior is byte-identical — fully backward-compatible. validate() rejects unknown formats.

The issue lists output_file: and a declared named-outputs: schema as alternatives — happy to rework toward either if you prefer that direction.

Coordination: aware of #2443 touching the same file; this diff is isolated to the structured-output addition and I'm happy to rebase in whichever order suits.

Testing

• Ran existing tests with uv sync && uv run pytest — full suite 3729 passed
• Four new tests in TestShellStep: data exposed on valid JSON; invalid stdout fails the step; no-flag backcompat (no data key); validate rejects unknown formats — the three behavior tests are 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 step-level tests)

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.