fix(frontmatter-read): put the frontmatter in the answer, not just a key count

`frontmatter_read` answered `Read frontmatter from <path> (N key(s))` and left
the frontmatter itself in `metadata["frontmatter"]`. MCP returns only
`response.answer`, so the LLM that asked for a file's frontmatter learned how
many keys exist and nothing else — the same shape of gap #379 describes for
`list`, where the caller cannot act on the result.

Render each key on its own line in the answer. Values are flattened to one
line each so a multi-line description cannot break the line-per-key layout:
lists join with commas and nested mappings render as compact JSON. An empty
frontmatter says `(no keys)` rather than trailing off after the colon.

The answer also names the probed path instead of the raw input, so a
suffix-less request reports the `.md` file that was actually read and the
caller can follow up with `read` without consulting metadata. Structured
`metadata["frontmatter"]` is unchanged, so programmatic consumers see no
difference.

Towards #379.

reme/steps/file_io/frontmatter_read.py

@@ -8,6 +8,7 @@ doesn't exist; otherwise ``{exists: true, frontmatter: {...}}``.
 ``path`` is a path relative to the workspace.
 """
 
+import json
 from pathlib import Path
 
 import frontmatter
@@ -18,6 +19,28 @@ from ..base_step import BaseStep
 from ...components import R
 
 
+def _render_value(value) -> str:
+    """Flatten one frontmatter value onto a single answer line."""
+    if isinstance(value, (list, tuple)):
+        rendered = ", ".join(str(item) for item in value)
+    elif isinstance(value, dict):
+        rendered = json.dumps(value, ensure_ascii=False, sort_keys=True)
+    else:
+        rendered = str(value)
+    return " ".join(rendered.split())
+
+
+def _render_frontmatter(meta: dict) -> list[str]:
+    """Render each frontmatter key on its own ``- key: value`` line.
+
+    ``frontmatter_read`` is served straight to the LLM, so the answer has to
+    carry the frontmatter itself; a key count alone leaves the caller with
+    nothing to act on. Values are flattened to one line each so a multi-line
+    description cannot break the line-per-key layout.
+    """
+    return [f"- {key}: {_render_value(value)}" for key, value in meta.items()]
+
+
 @R.register("frontmatter_read_step")
 class FrontmatterReadStep(BaseStep):
     """Read a Markdown file's frontmatter (YAML metadata only)."""
@@ -63,6 +86,8 @@ class FrontmatterReadStep(BaseStep):
             self.logger.info(f"[{self.name}] path={path} parse_error={exc!r}")
             return
         self.context.response.success = True
-        self.context.response.answer = f"Read frontmatter from {path} ({len(meta)} key(s))"
+        header = f"Read frontmatter from {probed} ({len(meta)} key(s))"
+        rendered = _render_frontmatter(meta)
+        self.context.response.answer = f"{header}:\n" + "\n".join(rendered) if rendered else f"{header}: (no keys)"
         self.context.response.metadata.update({"path": path, "exists": True, "frontmatter": meta, **resolved})
         self.logger.info(f"[{self.name}] path={path} keys={len(meta)}")

tests/unit/test_frontmatter_steps.py

@@ -192,3 +192,48 @@ async def test_explicit_md_path_has_no_resolved_path():
         assert resp.success is True
         assert "resolved_path" not in resp.metadata
         await store.close()
+
+
+@pytest.mark.asyncio
+async def test_read_answer_carries_the_frontmatter_itself():
+    """The answer must contain the frontmatter, not just a key count.
+
+    ``frontmatter_read`` is served straight to the LLM and MCP returns only
+    ``response.answer``, so a count alone leaves the caller with nothing to
+    act on. The suffix-less substitution is disclosed in the answer too.
+    """
+    with tempfile.TemporaryDirectory() as tmp, temp_chdir(tmp):
+        _seed(Path(tmp), NOTE, BODY)
+        store = await _make_store()
+        resp = await _run(FrontmatterReadStep, store, path="notes/post")
+        assert resp.success is True
+        assert resp.answer == "Read frontmatter from notes/post.md (2 key(s)):\n- name: n\n- tags: a"
+        assert resp.metadata["frontmatter"] == {"name": "n", "tags": ["a"]}
+        await store.close()
+
+
+@pytest.mark.asyncio
+async def test_read_answer_flattens_values_and_reports_empty():
+    """Multi-line, list and nested values stay on one line each; empty says so."""
+    with tempfile.TemporaryDirectory() as tmp, temp_chdir(tmp):
+        _seed(
+            Path(tmp),
+            "notes/rich.md",
+            "---\nname: Cobalt\ndescription: line one\n  line two\ntags:\n- a\n- b\nnested:\n  x: 1\n---\nbody\n",
+        )
+        _seed(Path(tmp), "notes/empty.md", "---\n---\nbody\n")
+        store = await _make_store()
+
+        resp = await _run(FrontmatterReadStep, store, path="notes/rich.md")
+        assert resp.answer == (
+            "Read frontmatter from notes/rich.md (4 key(s)):\n"
+            "- name: Cobalt\n"
+            "- description: line one line two\n"
+            "- tags: a, b\n"
+            '- nested: {"x": 1}'
+        )
+
+        resp = await _run(FrontmatterReadStep, store, path="notes/empty.md")
+        assert resp.answer == "Read frontmatter from notes/empty.md (0 key(s)): (no keys)"
+        assert resp.metadata["frontmatter"] == {}
+        await store.close()