dx: add consistent error messages with error codes to storage engine functions

[George-iam]

The storage engine (src/storage/engine.ts) functions like atomicWrite, appendLine, and writeJson catch and re-throw errors, but the error messages do not include the file path that failed or a consistent error code prefix. When a storage operation fails deep in the call stack, the error message should make it immediately clear which file and which operation failed. Add an AXME_STORAGE prefix and include the file path in all re-thrown errors.

Relevant files:

• src/storage/engine.ts — wrap caught errors with contextual messages

Example improvement:

// Before:
throw err;

// After:
throw new Error(`AXME_STORAGE: atomicWrite failed for ${filePath}: ${(err as Error).message}`, { cause: err });

Acceptance criteria:

• All functions that can throw (atomicWrite, appendLine, writeJson) include the file path and operation name in the error message
• Error messages have a consistent AXME_STORAGE: prefix
• Original error is preserved as cause
• npm run lint and npm test pass