feat: add OpenCode, Slate, and Cursor host configs

Three new hosts added to the declarative config system. Each is a typed
HostConfig object with paths, frontmatter rules, and path rewrites.
All generate valid SKILL.md output with zero .claude/skills path leakage.

- hosts/opencode.ts: OpenCode (opencode.ai), skills at ~/.config/opencode/
- hosts/slate.ts: Slate (Random Labs), skills at ~/.slate/
- hosts/cursor.ts: Cursor, skills at ~/.cursor/
- .gitignore: add .kiro/, .opencode/, .slate/, .cursor/, .openclaw/

Zero code changes needed — just config files + re-export in index.ts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -7,6 +7,11 @@ bin/gstack-global-discover
 .claude/skills/
 .agents/
 .factory/
+.kiro/
+.opencode/
+.slate/
+.cursor/
+.openclaw/
 .context/
 extension/.auth.json
 .gstack-worktrees/