feat: add multi-platform plugin support and session-start hook

Add support for 5 platforms beyond NPX:

- Claude Code: .claude-plugin/plugin.json + marketplace.json
- Cursor: .cursor-plugin/plugin.json + hooks-cursor.json
- Codex CLI: .codex/INSTALL.md with symlink setup
- OpenCode: .opencode/INSTALL.md with plugin config
- Gemini CLI: gemini-extension.json + GEMINI.md

Infrastructure:
- SessionStart hook with platform detection (Claude Code,
  Cursor, Copilot CLI) that injects available masters list
- Cross-platform run-hook.cmd polyglot wrapper (Windows + Unix)
- .version-bump.json for unified version management across
  all platform config files
- Update README/README_EN with multi-platform install instructions

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.opencode/INSTALL.md

@@ -0,0 +1,46 @@
+# Installing Master-skill for OpenCode
+
+## Quick Install
+
+Add to your `opencode.json`:
+
+```json
+{
+  "plugin": ["master-skill@git+https://github.com/xr843/Master-skill.git"]
+}
+```
+
+OpenCode will automatically install and register all skills.
+
+## Pin a Version
+
+```json
+{
+  "plugin": ["master-skill@git+https://github.com/xr843/Master-skill.git#v0.3.0"]
+}
+```
+
+## Available Skills After Install
+
+- `/xuanzang` — Xuanzang (Yogacara)
+- `/kumarajiva` — Kumarajiva (Madhyamaka)
+- `/huineng` — Huineng (Chan/Zen)
+- `/zhiyi` — Zhiyi (Tiantai)
+- `/fazang` — Fazang (Huayan)
+- `/yinguang` — Yinguang (Pure Land)
+- `/ouyi` — Ouyi (Tiantai-Pure Land)
+- `/xuyun` — Xuyun (Chan, Five Schools)
+- `/compare-masters` — Multi-tradition comparison
+- `/create-master` — Generate new master
+
+## Tool Mapping
+
+| Skill references | OpenCode equivalent |
+|---|---|
+| `Read` | `read_file` |
+| `Write` | `write_file` |
+| `Edit` | `edit_file` |
+| `Bash` | `shell` |
+| `Grep` | `grep` |
+| `Glob` | `glob` |
+| `Skill` | `skill` (native) |