agent: Send stale file notifications using the project_notifications tool (#34005)

This commit introduces the `project_notifications` tool, which
proactively pushes notifications to the agent.

Unlike other tools, `Thread` automatically invokes this tool on every
turn, even when the LLM doesn't ask for it. When notifications are
available, the tool use and results are inserted into the thread,
simulating an LLM tool call.

As with other tools, users can disable `project_notifications` in
Profiles if they do not want them.

Currently, the tool only notifies users about stale files: that is,
files that have been edited by the user while the agent is also working
on them. In the future, notifications may be expanded to include
compiler diagnostics, long-running processes, and more.

Release Notes:

- Added `project_notifications` tool

crates/assistant_tools/src/project_notifications_tool/description.md

@@ -0,0 +1,3 @@
+This tool reports which files have been modified by the user since the agent last accessed them.
+
+It serves as a notification mechanism to inform the agent of recent changes. No immediate action is required in response to these updates.

crates/assistant_tools/src/project_notifications_tool/prompt_header.txt

@@ -0,0 +1,3 @@
+[The following is an auto-generated notification; do not reply]
+
+These files have changed since the last read: