Add `--rename-only` and `--content-only` scope flags

[dolph]

Problem

Today find-replace always does both:

1. Rewrites file contents containing the pattern
2. Renames files and directories whose names contain the pattern

There's no way to scope to just one half. Users hit this when:

• Refactoring import paths where the filename happens to contain the same string but shouldn't be renamed (--content-only)
• Reorganizing files by naming convention without touching contents (--rename-only)
• Auditing the impact of one half before running the other

Proposed change

Add two mutually exclusive flags:

• --content-only — rewrite file contents; do not rename any files or directories
• --rename-only — rename files and directories; do not rewrite any contents

Default behavior is unchanged (both happen). Passing both flags is an error.

Acceptance

• --content-only skips the rename phase entirely
• --rename-only skips the content-rewrite phase entirely
• Passing both flags exits non-zero with a clear message
• Default behavior (neither flag) is unchanged
• README updated with examples for each
• Tests cover all three combinations on a tree containing both matching contents and matching names