fix: keep long frontmatter values on a single line (#3989)

`CommandRegistrar.render_frontmatter` calls `yaml.dump()` without `width=`,
so PyYAML applies its default ~80-column wrap and folds any long scalar onto
a continuation line.

A `description` longer than roughly 80 characters is therefore rendered as:

    ---
    name: speckit-implement
    description: Execute the implementation plan by processing and executing all tasks
      defined in tasks.md
    ---

The YAML remains valid and round-trips faithfully through `yaml.safe_load`,
so this is not data loss. It is a shape inconsistency with real consequences:

- Hand-written core command templates always keep `description` on one line,
  so preset- and extension-rendered commands do not match the files they sit
  beside in the same directory.
- Consumers that read frontmatter line-wise rather than with a YAML parser
  see the description truncated at the fold, followed by a stray line. Spec
  Kit itself hand-builds SKILL.md frontmatter in the skills path (see #3391),
  so this is not a hypothetical class of consumer.
- `speckit.implement`'s own description is 89 characters, so a preset that
  overrides it hits this immediately.

`width=float("inf")` disables the line-wrapping only; escaping, quoting and
the handling of genuinely multi-line values are unchanged, since PyYAML
selects the scalar style before applying width.

Adds a regression test that fails without the change.

Verified against the repo's own suite: 6354 passed. Four failures in
tests/integrations/test_integration_subcommand.py are present on a clean
checkout too (ANSI escapes in captured output) and are unrelated.

src/specify_cli/agents.py

@@ -157,7 +157,11 @@ class CommandRegistrar:
             return ""
 
         yaml_str = yaml.dump(
-            fm, default_flow_style=False, sort_keys=False, allow_unicode=True
+            fm,
+            default_flow_style=False,
+            sort_keys=False,
+            allow_unicode=True,
+            width=float("inf"),
         )
         return f"---\n{yaml_str}---\n"
 

tests/test_extensions.py

@@ -17,6 +17,7 @@ import platform
 import tempfile
 import shutil
 import tomllib
+import yaml
 from contextlib import contextmanager
 from pathlib import Path
 from datetime import datetime, timezone
@@ -2996,6 +2997,30 @@ Real body starts here.
         assert "Prüfe Konformität" in output
         assert "\\u" not in output
 
+    def test_render_frontmatter_keeps_long_description_on_one_line(self):
+        """A long description must not be folded across lines.
+
+        PyYAML wraps plain scalars at ~80 columns by default, which splits a
+        long ``description`` onto a continuation line. The YAML stays valid,
+        but the rendered frontmatter then differs in shape from the
+        hand-written core command templates, where ``description`` is always a
+        single line -- and consumers that read frontmatter line-wise see a
+        truncated description followed by a stray line.
+        """
+        long_description = (
+            "Execute the implementation plan by processing and executing all "
+            "tasks defined in tasks.md"
+        )
+        frontmatter = {"name": "speckit-implement", "description": long_description}
+
+        registrar = CommandRegistrar()
+        output = registrar.render_frontmatter(frontmatter)
+
+        assert f"description: {long_description}\n" in output
+
+        body = output.split("---\n")[1]
+        assert yaml.safe_load(body)["description"] == long_description
+
     def test_adjust_script_paths_does_not_mutate_input(self):
         """Path adjustments should not mutate caller-owned frontmatter dicts."""
         from specify_cli.agents import CommandRegistrar as AgentCommandRegistrar