fix(skills): toggling disable-model-invocation on an explicit false value corrupts SKILL.md

[hobostay]

Problem

PATCH /api/skills decides whether the disable-model-invocation key already exists with Boolean(frontmatter[key]) (app/api/skills/route.ts). A SKILL.md that explicitly declares the key as false:

---
name: my-skill
disable-model-invocation: false
---

is treated as having no key at all. Clicking "disable model invocation" in the UI then prepends a second disable-model-invocation: true line, and things go from bad to worse:

1. The yaml parser rejects duplicate keys outright (Map keys must be unique), so parseFrontmatter now throws for the whole file.
2. The pi skill loader catches that throw and returns skill: null — the skill disappears from the agent's skill list (and from the skills panel) after a single UI click.
3. The PATCH route itself calls parseFrontmatter before editing, so every further toggle attempt on that file returns 500 — the corruption cannot be undone from the UI, only by hand-editing the file.

Repro:

echo '---
name: t
description: t
disable-model-invocation: false
---
body' > SKILL.md
# toggle disable-model-invocation ON via the skills panel
# → file now starts with a duplicate key and no longer parses

Fix

• Extract the edit into lib/skill-frontmatter.ts (setDisableModelInvocation) with unit tests, and detect the key by presence (hasOwnProperty) instead of truthiness, updating the existing line in place.
• Restrict the edit to the frontmatter block, so a body line that merely documents the key is never touched.
• On removal, drop the line together with its preceding newline so no stray blank line is left inside the frontmatter.

Tests: node --experimental-strip-types --test lib/skill-frontmatter.test.mjs (8 cases, including the explicit-false regression and duplicate-key guards). Full suite 595/595, tsc --noEmit and eslint clean.

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