fix: prevent TUI gateway stdin EOF crash across all TUI-context subprocess calls

When Hermes runs in TUI mode, the gateway child process communicates with
the Node.js parent over a JSON-RPC protocol on stdin. Subprocess calls that
inherit this stdin fd can trigger a race condition where the child's stdin
read returns EOF, causing the gateway to exit cleanly (exit code 0) mid-tool-
execution.

This is the same root cause as issue #14036 (byterover plugin) and PR #39257
(SSH environment backend). This commit applies the fix — stdin=subprocess.DEVNULL
— to all 85 subprocess.run() and subprocess.Popen() calls that execute inside
the TUI gateway child process.

Scope: TUI-context code only (agent/, tools/, plugins/, tui_gateway/server.py).
CLI code (cli.py, hermes_cli/), tests, scripts, and gateway process management
are excluded — they don't run inside the TUI child and inherit the terminal's
stdin, not the JSON-RPC pipe.

85 call sites across 28 files. All files pass syntax check.

tools/terminal_tool.py

@@ -2436,13 +2436,13 @@ def check_terminal_requirements() -> bool:
             if not docker:
                 logger.error("Docker executable not found in PATH or common install locations")
                 return False
-            result = subprocess.run([docker, "version"], capture_output=True, timeout=5)
+            result = subprocess.run([docker, "version"], capture_output=True, timeout=5, stdin=subprocess.DEVNULL)
             return result.returncode == 0
 
         elif env_type == "singularity":
             executable = shutil.which("apptainer") or shutil.which("singularity")
             if executable:
-                result = subprocess.run([executable, "--version"], capture_output=True, timeout=5)
+                result = subprocess.run([executable, "--version"], capture_output=True, timeout=5, stdin=subprocess.DEVNULL)
                 return result.returncode == 0
             return False