fix: improve process tool descriptions to prefer foreground execution (#23395)

The tool descriptions pushed agents toward backgrounding anything over
5 seconds, including builds, tests, and installs where you actually
want to wait for the result. This led to unnecessary process_output
round-trips and missed the foreground timeout-to-reattach workflow
entirely.

Reframe background mode as the exception (persistent processes with
no natural exit) and foreground with an appropriate timeout as the
default. Replace "background process" with "tracked process" in
process_output, process_list, and process_signal since they work on
all tracked processes regardless of how they were started.

docs/ai-coder/agents/architecture.md

@@ -132,10 +132,10 @@ They traverse the same Tailnet tunnel used by web terminals and IDE connections.
 | `read_file`      | Reads file contents with line-number pagination.                   |
 | `write_file`     | Writes content to a file.                                          |
 | `edit_files`     | Performs atomic search-and-replace edits across one or more files. |
-| `execute`        | Runs a shell command (foreground or background).                   |
-| `process_output` | Retrieves output from a background process.                        |
+| `execute`        | Runs a shell command, waiting for completion up to a timeout.      |
+| `process_output` | Retrieves output from a tracked process.                           |
 | `process_list`   | Lists all tracked processes in the workspace.                      |
-| `process_signal` | Sends a signal (SIGTERM or SIGKILL) to a background process.       |
+| `process_signal` | Sends a signal (SIGTERM or SIGKILL) to a tracked process.          |
 
 ### Platform tools