agent: Push diffs of user edits to the agent (#34487)

This change improves user/agent collaborative editing.

When the user edits files that are used by the agent, the
`project_notification` tool now pushes *diffs* of the changes, not just
file names. This helps the agent to stay up to date without needing to
re-read files.

Release Notes:

- Improved user/agent collaborative editing: agent now receives diffs of
user edits

Cargo.lock

@@ -745,6 +745,7 @@ dependencies = [
  "futures 0.3.31",
  "gpui",
  "icons",
+ "indoc",
  "language",
  "language_model",
  "log",