Refactor ZenFsAdapter to be lean and slick

[pyramation]

Summary

Refactored the ZenFsAdapter from 186 lines to 47 lines (75% reduction) by delegating to zenfs.fs.promises instead of manually wrapping sync methods. The adapter now only provides the minimal glue needed for just-bash compatibility:

• Stat shape conversion (zen-fs returns bigint for mode/size, just-bash expects number)
• resolvePath() and getAllPaths() which are just-bash-specific methods
• Return type adjustments (mkdir/mv return void, not string)

Removed custom type definitions, manual recursive cp implementation, and the ensureParentDirs helper.

Review & Testing Checklist for Human

• Verify ensureParentDirs removal is acceptable: The old adapter auto-created parent directories on write operations. The new adapter does not - tests work because they use explicit mkdir -p. Confirm this behavioral change is intentional for the demo.
• Check error handling parity: The old adapter had custom try/catch for rm --force. The new one delegates directly to fs.promises.rm. Verify edge cases still work as expected.
• Confirm interface compatibility: The adapter no longer explicitly implements IFileSystem - it relies on duck typing. Run through the demo scenarios to ensure all operations work.

Test plan:

cd packages/zen-bash
pnpm test

All 9 tests should pass, demonstrating write-via-bash/read-via-zenfs and vice versa.

Notes

The getAllPaths() method still uses sync methods (readdirSync, lstatSync) since it returns string[] synchronously - this is required by just-bash's interface for glob matching.

Link to Devin run: https://app.devin.ai/sessions/e5a59ab234c5481e817dad43a74cfc6e
Requested by: Dan Lynch (@pyramation)

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