feat: convert opencode-ast to an MCP server

[wj-e2b]

Summary

• replace the OpenCode plugin entrypoint with a stdio MCP server exposing the existing ast operations
• add workspace-root containment for absolute paths, traversal, and symlink escapes
• package an executable Bun bundle and document generic MCP client configuration
• enforce a 14-day minimum dependency age for Bun installs

Verification

• bun test — 87 passed
• bun x tsc --noEmit
• bun run bundle
• bundled stdio MCP listTools + callTool smoke test
• clean npm pack --dry-run --json via prepack
• independent pre-commit review passed with no security or logic findings