fix(build): quote YAML descriptions containing ': ', trailing ':', or reserved indicators

[mvanhorn]

Summary

Closes #102.

`generateYamlFrontmatter` (scripts/lib/utils.js) only quoted values beginning with `[` or `{`, so scalars containing a colon-space were written unquoted. Strict YAML parsers, including Codex's skill loader and GitHub's frontmatter preview, then rejected the emitted file with `mapping values are not allowed in this context` and skipped loading the skill. The `harden` skill's description, which reads `Make interfaces production-ready: error handling, ...`, hit this path; every generated provider copy (`.codex`, `.claude`, `.cursor`, `.gemini`, `.agents`, `.kiro`, `.opencode`, `.pi`, `.rovodev`, `.trae-cn`, `.trae`) ended up with an invalid front matter block even though `source/skills/harden/SKILL.md` was already quoted.

Broaden `needsQuoting` to cover the documented YAML 1.2 failure modes rather than only the flow-indicator case:

• values starting with a flow indicator (`[`, `{`) or a reserved indicator (`#`, `&`, `*`, `!`, `|`, `>`, `%`, `@`, backtick), leading whitespace, a leading quote, or a leading `-`
• values containing `: ` or ending in `:` (the issue-reported regression)
• values containing ` #` (inline comment marker)

Type of change

• Bug fix
• Build system / tooling

Verification

```console
$ bun test tests/lib/utils.test.js
64 pass
0 fail
```

New tests cover the colon-space regression, trailing colon, leading reserved indicators, and mid-string ` #` sequence, each paired with a `parseFrontmatter` roundtrip so the quoted output recovers the original scalar.

`bun run build` regenerates all 11 provider copies. The harden SKILL.md front matter is now valid under strict YAML parsers:

```yaml
description: "Make interfaces production-ready: error handling, empty states, onboarding flows, i18n, text overflow, and edge case management. Use when the user asks to harden, make production-ready, handle edge cases, add error states, design empty states, improve onboarding, or fix overflow and i18n issues."
```

Checklist

• Source files updated in `source/` (no change - already quoted; build-side regression)
• `bun run build` ran successfully
• `bun test` passes (64/64 in tests/lib/utils.test.js)
• Tested with at least one provider (Codex loader path simulated in test via `parseFrontmatter` roundtrip)
• README / DEVELOP.md updated if needed (not needed - internal build fix)

---

Note

Low Risk
Low risk: limited to build-time frontmatter stringification plus added unit tests; behavioral change is broader quoting which may slightly alter emitted YAML formatting but should improve parser compatibility.

Overview
Fixes a build-time YAML frontmatter escaping bug. generateYamlFrontmatter now quotes string scalars that would be misparsed by strict YAML parsers (e.g., values containing : , ending with :, containing #, or starting with YAML-reserved indicators/whitespace/quotes).

Regenerates provider skill copies (e.g., harden descriptions) to include quoted descriptions, and adds regression tests covering the new quoting cases with parse round-trips.

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