fix: avoid Bun file IO in morph_edit

[fengjikui]

Summary

Fixes #29.

morph_edit now uses Node fs/promises for local file reads and writes instead of Bun.file / Bun.write. This keeps the Morph SDK calls, tool names, WarpGrep tools, and compaction hooks unchanged while allowing the plugin to run in OpenCode Desktop / Node-style plugin runtimes that do not expose a global Bun object.

Root cause

The plugin is built as a JavaScript package for OpenCode, but the morph_edit implementation directly referenced Bun runtime globals for file IO. In OpenCode Desktop, the plugin can execute in an Electron/Node environment where Bun is not defined, so morph_edit could fail before reaching the Morph API.

Changes

• Replace Bun.file(...).text() with readFile(..., "utf8").
• Replace Bun.write(...) with writeFile(..., "utf8").
• Preserve the existing missing-file behavior for complete new-file content vs. lazy-marker edits.
• Add a regression test that keeps morph_edit file IO from reintroducing Bun.file / Bun.write.

Validation

• bun test index.test.ts -t 'morph_edit'
• bun test
• bun run typecheck
• bun run build
• bun run ci
• npm pack --dry-run
• git diff --check