feat(harness): authority, turn contract, tools, clean_agent_preview

Chat/agent harness work: request-authority model, turn contract, tool
policy/parsing/execution, clean_agent_preview streaming/thinking, ask_user
persistence, session probes, and scheduler.

Source: vendor/photocraft (ad1cff44) harness files, 3-way merged onto
upstream/dev (a8c147b2). Harness core files were independently developed
in both lineages; resolution policy documented in pr_work/C_conflicts.md
(vendor kept where ahead, upstream kept where vendor is behind; full delta
preserved in pr_work/C_upstream_delta.patch).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_markdown_lazy_lib_loading_js.py

@@ -368,7 +368,7 @@ def test_detached_container_math_typesets_with_the_real_renderer(node_available)
 
     mdToHtml defers math to a document-scoped flush, which cannot reach a
     detached node, so the export has to typeset its own container before
-    handing it to browser printing. This is that container: pending spans in, real
+    handing it to html2pdf. This is that container: pending spans in, real
     KaTeX markup out, no .katex-error and nothing left pending.
     """
     out = _run_node(
@@ -400,7 +400,7 @@ def test_detached_container_math_typesets_with_the_real_renderer(node_available)
     assert "ody-math-pending" not in out["written"]
 
 
-def test_pdf_export_typesets_its_container_before_print():
+def test_pdf_export_typesets_its_container_before_html2pdf():
     """Ordering in a call site, so pin the call site. No node needed."""
     source = document_source()
     match = re.search(r"\n  async function exportAsPdf\(\) \{(.*?)\n  \}\n", source, re.S)
@@ -410,7 +410,7 @@ def test_pdf_export_typesets_its_container_before_print():
     render = "await markdownModule.renderMath(container);"
     assert render in body, "the export never typesets its detached container"
     assert body.index("container.innerHTML = html;") < body.index(render)
-    assert body.index(render) < body.index("frame.contentWindow.print()")
+    assert body.index(render) < body.index("window.html2pdf()")
 
 
 def test_md_to_html_renders_inline_once_katex_is_loaded(node_available):