1Get output from a background shell process.
 2
 3<usage>
 4- Provide shell_id from a background bash execution
 5- Returns current stdout/stderr
 6- Shows if process is still running or completed
 7</usage>
 8
 9<behavior>
10- Returns cumulative output from process start
11- Check "done" field to see if process completed
12- Can call multiple times to see incremental output
13</behavior>
14
15<example>
16After starting a server with `run_in_background=true`:
17```
18shell_id: "abc123"
19```
20→ Returns server output and whether it's still running.
21</example>