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>

.claude-plugin/plugin.json

@@ -0,0 +1,22 @@
+{
+  "name": "master-skill",
+  "description": "Chinese Buddhist Master AI teaching personas — 8 prebuilt masters with CBETA-cited doctrinal responses, RAG-grounded in FoJin knowledge graph",
+  "version": "0.3.0",
+  "author": {
+    "name": "xr843",
+    "email": "xr843@users.noreply.github.com"
+  },
+  "homepage": "https://github.com/xr843/Master-skill",
+  "repository": "https://github.com/xr843/Master-skill",
+  "license": "MIT",
+  "keywords": [
+    "buddhism",
+    "chinese-buddhism",
+    "ai-persona",
+    "digital-humanities",
+    "rag",
+    "cbeta",
+    "fojin",
+    "agent-skills"
+  ]
+}