feat: contributor add-host skill + fix version sync

- contrib/add-host/SKILL.md.tmpl: contributor-only skill that guides
  new host config creation. Lives in contrib/, excluded from user installs.
- package.json: sync version with VERSION file (0.15.2.1)

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

contrib/add-host/SKILL.md.tmpl

@@ -0,0 +1,63 @@
+---
+name: gstack-contrib-add-host
+description: |
+  Contributor-only skill: create a new host config for gstack's multi-host system.
+  NOT installed for end users. Only usable from the gstack source repo.
+---
+
+# /gstack-contrib-add-host — Add a New Host
+
+This skill helps contributors add support for a new AI coding agent to gstack.
+
+## What you'll create
+
+A single TypeScript file in `hosts/<name>.ts` that defines:
+- CLI binary name for detection
+- Skill directory paths (global + local)
+- Frontmatter transformation rules
+- Path and tool rewrites
+- Runtime root symlink manifest
+
+## Steps
+
+### 1. Gather host info
+
+Ask the contributor:
+- What's the agent's name? (e.g., "OpenCode")
+- What's the CLI binary? (e.g., "opencode")
+- Where does it store skills globally? (e.g., "~/.config/opencode/skills/")
+- Where does it store skills locally in a project? (e.g., ".opencode/skills/")
+- What frontmatter fields does it support? (name + description is the minimum)
+- Does it have its own tool names? (e.g., "exec" instead of "Bash")
+
+### 2. Create the config file
+
+Use `hosts/opencode.ts` as a reference. Create `hosts/<name>.ts` with the
+gathered info. Follow the HostConfig interface in `scripts/host-config.ts`.
+
+### 3. Register in index
+
+Add the import and re-export in `hosts/index.ts`.
+
+### 4. Add to .gitignore
+
+Add `.<name>/` to `.gitignore`.
+
+### 5. Generate and verify
+
+```bash
+bun run gen:skill-docs --host <name>
+```
+
+Check:
+- Output exists at `.<name>/skills/gstack-*/SKILL.md`
+- No `.claude/skills` path leakage
+- Frontmatter matches expected format
+
+### 6. Run tests
+
+```bash
+bun test test/gen-skill-docs.test.ts
+```
+
+All parameterized tests auto-include the new host.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "gstack",
-  "version": "0.15.2.0",
+  "version": "0.15.2.1",
   "description": "Garry's Stack — Claude Code skills + fast headless browser. One repo, one install, entire AI engineering workflow.",
   "license": "MIT",
   "type": "module",