fix: add encoding='utf-8' to subprocess calls with text=True

[AlexFucuson9]

Bug

On non-UTF-8 Windows locales (e.g. CP936/GBK), subprocess.run/Popen with text=True but no explicit encoding crashes _readerthread when child outputs UTF-8 text:

UnicodeDecodeError: 'gbk' codec can't decode byte 0xae

This triggers cascading failure: pipe buffers fill → event loop stalls → ws write slow → TUI freezes → gateway restarts.

Fix

Add encoding='utf-8', errors='replace' to all 22 bare text=True subprocess calls across 13 files:

• tui_gateway/server.py (4)
• tui_gateway/git_probe.py (1)
• agent/lsp/install.py (3)
• agent/anthropic_adapter.py (2)
• agent/context_references.py (2)
• agent/secret_sources/bitwarden.py (2)
• agent/copilot_acp_client.py (1)
• agent/skill_preprocessing.py (1)
• agent/coding_context.py (1)
• agent/transports/codex_app_server.py (1)
• agent/shell_hooks.py (1)
• gateway/status.py (2)
• gateway/shutdown_forensics.py (1)

Fixes #53428

[teknium1]

Closing as superseded by the consolidated Windows console-flash work tracked in #54220.

The relevant pieces from this PR/cluster have now landed through the targeted follow-up PRs #54236, #53892, and #54417, or are recorded in the umbrella tracker for any remaining native-Windows verification. Keeping this separate PR open would duplicate the tracker and the merged follow-up work.

Thanks for digging into this — the reports and PRs in this cluster helped identify the remaining spawn legs.