fix: keep long frontmatter values on a single line

[deborre]

CommandRegistrar.render_frontmatter (src/specify_cli/agents.py) calls yaml.dump() without width=, so PyYAML applies its default ~80-column wrap and folds long scalars onto a continuation line.

A description longer than roughly 80 characters renders as:

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

Why this is worth fixing

The YAML stays valid and round-trips faithfully through yaml.safe_load, so this is not data loss — I want to be accurate about the severity. It is a shape inconsistency, with real consequences:

• Hand-written core command templates always keep description on a single line, so preset- and extension-rendered commands don't match the files sitting beside them 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 (SKILL.md frontmatter corrupts multiline descriptions and breaks on control characters #3391), so that's not a hypothetical class of consumer.
• speckit.implement's own description is 89 characters, so any preset overriding it hits this immediately.

The change

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

Verification

• Added a regression test that fails without the change (asserts the description survives on one line and still parses).
• Full suite: 6354 passed. Four failures in tests/integrations/test_integration_subcommand.py reproduce on a clean checkout of main as well (ANSI escapes in captured output) and are unrelated to this change.

Reproduction

specify preset add --dev <preset with an 89-char command description>
cat .claude/skills/speckit-<name>/SKILL.md   # description folded across two lines

[Copilot]

🟢 Ready to approve

The focused implementation correctly addresses the reported formatting inconsistency with regression coverage.

This review doesn't count toward merge requirements. Sign up for the private preview to control whether Copilot approvals count.

Pull request overview

Prevents PyYAML from wrapping long frontmatter values, preserving compatibility with line-oriented consumers.

Changes:

• Disables YAML width-based wrapping.
• Adds a round-trip regression test for long descriptions.

File summaries

File	Description
src/specify_cli/agents.py	Disables frontmatter scalar wrapping.
tests/test_extensions.py	Tests single-line output and YAML parsing.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced

We're testing this review assessment. Please use 👍 or 👎 to tell us if it's correct.

[mnriem]

Thank you!