Synchronize working directory files upon branch switch & protect uncommitted changes

[barissalihbabacan]

Problem

Calling switch_branch only updates .osmos/HEAD with the target branch name without updating files on disk. If the user creates a commit on the new branch, it incorporates leftover files from the previous branch.

Proposed Solution

1. Verify working directory is clean relative to current HEAD (or warn/stash).
2. Clean files present in current HEAD and write files from target branch HEAD commit.
3. Update .osmos/HEAD.

Acceptance Criteria

• Switching between branches replaces disk files with the target branch's snapshot.
• Prevents switching if dirty files would be overwritten.