Overhaul inline assistant (#12846)

This pull request introduces a new diff mechanism that helps users
understand exactly which lines were changed by the LLM.

Release Notes:

- N/A

crates/assistant/Cargo.toml

@@ -47,6 +47,7 @@ semantic_index.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 settings.workspace = true
+similar.workspace = true
 smol.workspace = true
 strsim = "0.11"
 strum.workspace = true