Follow-ups to the recently-landed agent terminal sandboxing work.

- Make the persistent "Allow Unsandboxed Terminal Commands" setting
(`allow_unsandboxed`) the single off-switch for the agent terminal
sandbox: when enabled, the sandboxed terminal tool isn't exposed and the
system prompt omits the sandbox section, so the model uses the plain
`terminal` tool (and on Windows, WSL sandbox setup is skipped). This
removes the dead, unwired `disabled` setting that was meant to do the
same thing but had no UI, writer, or docs. Per-command and per-thread
`unsandboxed: true` grants are unchanged.
- Expand the blocklist of Windows-specific environment variables that
aren't forwarded into the WSL sandbox (system locations, `HOME`/profile
paths, host/session identity, CPU descriptors, etc.) so they can't
shadow or break Linux commands. It stays a blocklist, so portable
variables like `LANG` still reach the command.

Release Notes:

- N/A