feat(workflows): add from_json expression filter

[doquanghuy]

Description

Fixes #2960.

Adds one arg-less pipe filter to the sandboxed expression evaluator:

items: "{{ steps.emit.output.stdout | from_json }}"

_filter_from_json parses a JSON string into its typed value (list/dict/scalar). Semantics are parse-or-raise: invalid JSON or non-string input raises a clear ValueError — never a silent passthrough, since a parse failure means the pipeline wiring is wrong and silence would hide it. The module docstring's filter list is updated; no other filter or evaluator behavior is touched.

This is the smallest unblock for feeding fan-out items: (or any condition/arg) from a step that computes a collection at runtime. It composes with — and doesn't preclude — a future declared-outputs: mechanism, which I'm proposing separately.

Testing

• Ran existing tests with uv sync && uv run pytest — tests/test_workflows.py 210 passed
• Three new tests in TestExpressions (valid JSON → typed value; invalid JSON raises; non-string raises) — all three are red against current main, green with the filter (verified both directions)
• uvx ruff check src/ — clean
• Tested locally with uv run specify --help
• Tested with a sample project (covered by the evaluator tests; exercised manually with the [Bug]: fan-out silently coerces a non-list 'items' to [] — zero-instance runs with no error #2956 repro workflow rewired through | from_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 a review — happy to adjust anything.