feat(hashline): add file deletion operation to hashline_edit

[randomm]

Problem

There is no mechanism to delete a file via hashline. With hashline as the sole file editing mechanism, agents cannot remove files during refactoring (e.g. deleting deprecated modules, cleaning up generated files).

Proposed Solution

Add a delete_file operation to hashline_edit:

{ "op": "delete_file" }

• Requires a prior hashline_read (same enforcement as other ops)
• Atomically deletes the file from disk
• Clears the file's entry from FileTime state

Acceptance Criteria

• hashline_edit with op: "delete_file" removes the file from disk
• Operation is rejected if hashline_read was not called first
• File deletion is reflected in FileTime state (subsequent edits correctly require a new read)
• Attempting to delete a non-existent file returns a clear error
• All existing hashline tests still pass
• Adversarial review: APPROVED

Quality Gates

• bun run typecheck — 0 errors
• bun test — 0 failures