Also demote privileges for text editing utils

[farrelmahaztra]

Note

Medium Risk
Touches subprocess creation and privilege/UID handling, which can affect sandboxing, permissions, and cross-platform behavior (especially root-in-container vs. Windows).

Overview
Introduces a shared get_demote_preexec_fn() in coding/utils.py to standardize subprocess isolation: on Unix it uses setsid and, when running as root, demotes to uid/gid 1000; on Windows it returns None.

Updates ClaudeBashSession (bash.py), the shared BashSession (session.py), and async read_file_async/write_file_async to use this helper instead of inline, platform-specific process creation logic, removing duplicated os/sys branching.

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