enhancement(#782): emit gsd skills to ~/.cline/skills for Cline >= v3.48 (#809)

Cline added a global skills system (~/.cline/skills/<name>/SKILL.md) in
v3.48.0, but gsd treated Cline as rules-only and emitted zero skills
(getGlobalSkillsBase('cline')=null, empty artifact kinds). This makes gsd
emit skills for Cline at global scope, alongside the existing .clinerules.

- runtime-homes: getGlobalSkillsBase('cline') -> ~/.cline/skills (was null)
- runtime-artifact-layout: cline emits a skills kind for GLOBAL scope only
  (local stays .clinerules-only), mirroring claude's scope dispatch
- install.js: convertClaudeCommandToClineSkill emits name+description-only
  SKILL.md frontmatter (Cline/agentskills.io spec; no Claude-specific
  allowed-tools/argument-hint/agent), hyphen-normalized + .cline/-rewritten
  body; global cline routed through the skills path while .clinerules is
  still written; _applyRuntimeRewrites cline case handles custom
  CLINE_CONFIG_DIR; convertClaudeToCliineMarkdown also rewrites bare
  ~/.claude and CLAUDE_CONFIG_DIR
- docs: install-on-your-runtime.md documents Cline global skills vs local rules
- tests: converter (name+description-only), global emission, skills+.clinerules
  coexistence, scope-aware layout, custom-dir paths, idempotency

Closes #782

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.changeset/782-cline-skills-emission.md

@@ -0,0 +1,5 @@
+---
+type: Changed
+pr: 809
+---
+**Cline global installs now emit skills, not just rules:** gsd writes skills to `~/.cline/skills/<name>/SKILL.md` for Cline ≥ v3.48.0 (see [Cline skills docs](https://docs.cline.bot/customization/skills)), in addition to the existing `.clinerules` file. Each `SKILL.md` carries `name`/`description` frontmatter (agentskills.io) with paths rewritten to the `.cline/` convention. Local installs remain `.clinerules`-only. The `.clinerules` rules file continues to be emitted for compatibility, and upgrading over an existing rules-only install emits the new skills on the next run.