Add recent_changes tool to surface unsaved buffer diffs as agent context

[BitStrider]

The agent currently has no way to know what files the user is actively
editing unless they explicitly @-mention them. This means the agent can
easily clobber unsaved work or miss important in-progress changes when
making edits.

This PR adds a recent_changes tool that lets the agent call a single
tool and immediately see every dirty buffer in the project as a unified
diff — what changed, in which file, since the last save.

The tool is read-only, always-on, and follows the exact same pattern as
the existing diagnostics and grep tools. No new architecture, no
feature flag needed.

Internally it snapshots dirty buffers synchronously via BufferStore,
then loads the saved-on-disk version asynchronously via Fs and runs
diffy::create_patch to produce the diff. Three cases are handled:
files modified since last save, files never saved yet, and files whose
saved version can't be read from disk.

Also promotes diffy to a workspace dependency since it was already
pinned inline in the language crate.

Release Notes:

• Added: recent_changes tool for the agent to discover unsaved edits
  across the project without requiring explicit file mentions