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