feat(workflows): add from_json expression filter (#2961)

* feat(workflows): add from_json expression filter

Step outputs captured as strings could never become typed values in
templates - the filter set was default/join/map/contains only, so e.g.
a fan-out items: could never consume a step's JSON stdout. Add an
arg-less from_json pipe filter with parse-or-raise semantics: invalid
JSON or non-string input raises a clear ValueError rather than passing
through silently.

Fixes #2960

* fix(expressions): make from_json strict — reject any arguments

Address review (#2961): from_json('x') and from_json() previously fell through to a silent passthrough of the unparsed value. Reject any parenthesized form with a clear error so mis-wired templates fail loudly. Rename test to ...parses_object (JSON under test is an object) and add coverage for the strict no-arguments behavior.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* docs(workflows): document the from_json expression filter

Address Copilot review: the user-facing filter references omitted the
newly added `from_json` filter. Add it to the ARCHITECTURE.md filter table
(with the `{{ steps.emit.output.stdout | from_json }}` example) and to the
filter enumerations in workflows/README.md and docs/reference/workflows.md
so the docs match the evaluator's capabilities.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(workflows): make from_json strictness reject trailing tokens; fix docstring

Address Copilot review:
- Strictness only rejected parenthesized forms, so typos like
  `| from_json)` or `| from_json extra` still fell through to the
  unknown-filter path and silently returned the unparsed value. Match on
  the leading filter token and require the whole filter to be exactly
  `from_json`, so every mis-wired form raises. Extend the rejection test to
  cover the trailing-token cases.
- The module docstring claimed "no imports", which is misleading now that
  the module imports `json`. Reword to state the actual sandbox guarantee:
  templates cannot do file I/O, import modules, or run arbitrary code.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

docs/reference/workflows.md

@@ -280,7 +280,7 @@ Steps can reference inputs and previous step outputs using `{{ expression }}` sy
 | `steps.specify.output.file`    | Output from a previous step          |
 | `item`                         | Current item in a fan-out iteration  |
 
-Available filters: `default`, `join`, `contains`, `map`.
+Available filters: `default`, `join`, `contains`, `map`, `from_json`.
 
 Example: