feat: add fromjson template filter for consuming MCP tool results

[hertznsk]

Closes #579.

Summary

MCP steps (type: mcp) return the raw MCP envelope, and many widely used MCP servers return data only as a JSON string in content[0].text and never populate structuredContent. Jinja2 ships tojson but no inverse, and TemplateRenderer registered only the json (dumps) and default filters — so downstream steps could not parse a previous MCP tool's JSON text inside arguments, routes, or stdin templates, blocking deterministic read-modify-write workflows (the issue's GitLab update_merge_request label-merge chain is the concrete case).

This PR registers a built-in fromjson filter — the inverse of the existing json filter:

labels: "{{ (((steps.get_mr.output.content[0].text | fromjson).labels | default([])) + ['Conductor::Need human']) | unique | list | tojson }}"

On non-string input or invalid JSON it raises TemplateError with a specific suggestion, consistent with the renderer's error contract.

One extra fix found during review

TemplateRenderer.render() re-wrapped any TemplateError raised by a filter under its generic except Exception handler, replacing the filter's specific suggestion with "Check template and context for errors" and burying the real cause. render() now re-raises filter-raised TemplateErrors unchanged (attaching the rendered template for context when missing), so the filter's own error contract reaches the caller. Regression tests pin the structured contract (suggestion, template_string, and the JSONDecodeError cause).

Changes

• src/conductor/executor/template.py — fromjson filter registration + implementation; TemplateError-preserving branch in render()
• tests/test_executor/test_template.py — 6 unit tests: object/array/scalar parsing, composition with default + concat, both error paths with the structured contract asserted
• tests/test_executor/test_mcp_step.py — integration regression: a prior MCP step's content[0].text parsed with fromjson, merged, serialized with tojson, arriving at the tool as a native list via argument auto-coercion (the issue's exact pattern)
• docs/workflow-syntax.md — MCP Steps section documents parsing content[0].text with fromjson when a server returns no structuredContent
• plugins/conductor/skills/conductor/references/{yaml-schema,authoring}.md — fromjson added to both ### Filters blocks
• changelog.d/579.added.md — towncrier fragment

Verification

• tests/test_executor/ + tests/test_config/: 2172 passed
• Full suite (-n 4): 11024 passed; the single failure (tests/test_cli/test_status.py::TestTerminalCostCellBilling::test_status_table_renders_the_label) is a pre-existing parallel-isolation flake — it fails identically on clean main under the same command and passes in isolation with and without this change
• make check (ruff + ty): clean

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Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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[Jason Robert (jrob5756)]

Thanks for adding the fromjson filter for MCP tool results. I have one recommended clarification about passing parsed collections to MCP arguments and route conditions.

1 approved findings: 1 inline, 0 in this review body.

[Jason Robert (jrob5756)]

LGTM, thanks for adding the fromjson template filter to consume MCP tool results!