tests(init): add error-path coverage for init command

[jikongabc]

What

Add error-path tests for the init command, covering:

• repository path is a file
• parent directory is not writable
• --bare init on an existing repository

Why

These cases previously lacked coverage and could regress silently.
Adding explicit tests ensures init fails with clear and expected errors.

How

Implemented integration tests in tests/command/init_test.rs
following existing test patterns and using serial execution where required.