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/memory/hindsight/__init__.py

@@ -695,6 +695,7 @@ class HindsightMemoryProvider(MemoryProvider):
                 subprocess.run(
                     [uv_path, "pip", "install", "--python", sys.executable, "--quiet", "--upgrade"] + deps_to_install,
                     check=True, timeout=120, capture_output=True,
+                    stdin=subprocess.DEVNULL,
                 )
                 print("  ✓ Dependencies up to date")
             except Exception as e:
@@ -1101,6 +1102,7 @@ class HindsightMemoryProvider(MemoryProvider):
                             [uv_path, "pip", "install", "--python", sys.executable,
                              "--quiet", "--upgrade", f"hindsight-client>={_MIN_CLIENT_VERSION}"],
                             check=True, timeout=120, capture_output=True,
+                            stdin=subprocess.DEVNULL,
                         )
                         logger.info("hindsight-client upgraded to >=%s", _MIN_CLIENT_VERSION)
                     except Exception as e: