Add first-class Grok Build harness support

Emit .grok skills, agents, and PostToolUse/Stop hooks; wire the CLI
installer and downloads; fix the plugin install path to #plugin; and
document Grok in HARNESSES.md and README.

AI assistance: written with Grok Build.

.github/ISSUE_TEMPLATE/feature_request.md

@@ -21,6 +21,7 @@ assignees: ''
 - [ ] Gemini CLI
 - [ ] Codex CLI
 - [ ] VS Code Copilot
+- [ ] Grok Build
 - [ ] Kiro
 - [ ] OpenCode
 - [ ] Qoder

.github/PULL_REQUEST_TEMPLATE.md

@@ -26,7 +26,7 @@ This repo is issue-first for outside contributions. If you are not `pbakaus` or
 - [ ] Source files updated in `source/`
 - [ ] `bun run build` ran successfully
 - [ ] `bun test` passes
-- [ ] Tested with at least one provider (Cursor / Claude Code / Gemini CLI / Codex / Copilot / Kiro / OpenCode / Qoder / Mistral Vibe)
+- [ ] Tested with at least one provider (Cursor / Claude Code / Gemini CLI / Codex / Copilot / Grok Build / Kiro / OpenCode / Qoder / Mistral Vibe)
 - [ ] README / DEVELOP.md updated if needed
 - [ ] I reviewed the full diff myself before requesting human review
 - [ ] I disclosed any AI assistance in this PR and related commits/comments, or no AI assistance was used

.github/workflows/sync-generated-output.yml

@@ -26,6 +26,7 @@ env:
     .cursor
     .gemini
     .github/skills
+    .grok
     .kiro
     .opencode
     .pi