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.

plugins/google_meet/audio_bridge.py

@@ -86,6 +86,7 @@ class AudioBridge:
                         ["pactl", "unload-module", str(mod_id)],
                         check=False,
                         capture_output=True,
+                        stdin=subprocess.DEVNULL,
                     )
                 except Exception:
                     # Best-effort teardown — never raise from here.
@@ -111,6 +112,7 @@ class AudioBridge:
                 check=True,
                 capture_output=True,
                 text=True,
+                stdin=subprocess.DEVNULL,
             )
         except FileNotFoundError as exc:
             raise RuntimeError(
@@ -135,6 +137,7 @@ class AudioBridge:
                 check=True,
                 capture_output=True,
                 text=True,
+                stdin=subprocess.DEVNULL,
             )
         except subprocess.CalledProcessError as exc:
             # Roll back the null-sink we just created so we don't leak it.
@@ -142,6 +145,7 @@ class AudioBridge:
                 ["pactl", "unload-module", str(sink_mod_id)],
                 check=False,
                 capture_output=True,
+                stdin=subprocess.DEVNULL,
             )
             raise RuntimeError(
                 f"pactl load-module virtual-source failed: {exc.stderr or exc}"