chore(cleanup): remove terraphim_repl crate

Remove the terraphim_repl crate which is superseded by terraphim_agent
with --features repl-full. The crate was already excluded from the
workspace; this completes the removal by deleting the directory.

Verifications:
- cargo check --workspace passes
- cargo build -p terraphim_agent --features repl-full works
- cargo build -p terraphim-cli works

Part of: #624

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: AlexMikhalev