fix: document ferrflow CLI dependency for dry_run and validate_config tools

[BryanFRD]

Describe the bug
The `dry_run` and `validate_config` tools shell out to the `ferrflow` CLI binary, but the README doesn't mention this requirement.

Users who install the MCP server via npm will hit cryptic errors when these tools can't find the binary. Add installation prerequisites to the README and return a clear error message when ferrflow is not found.

Package
Which package is affected? [x] server [x] tools [ ] docs

To Reproduce
Not provided in the original issue — to be filled in.

Expected behavior
Not specified explicitly — see description.

Environment

• Package version: main
• Browser/OS (if frontend): n/a