fix(cosh-ng): suppress bashdb warning on Alibaba Cloud Linux

[kongche-jbw]

Description

On Alibaba Cloud Linux, /etc/sysconfig/bash-prompt-history detects
shopt extdebug and attempts to load /usr/share/bashdb/bashdb-main.inc,
producing a warning when bashdb is not installed. This happens because
cosh-ng enables extdebug for DEBUG trap interception, and system scripts
in PROMPT_COMMAND see it on subsequent prompt draws.

Fix: disable extdebug at the end of preexec (before command runs) and
prepend a suspend helper in PROMPT_COMMAND, then re-enable in precmd
(which runs last). System scripts now execute with extdebug off — their
normal expected environment — while cosh's intercept still works.

Related Issue

no-issue: environment-specific warning fix with no tracking issue

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional change)
• Performance improvement
• CI/CD or build changes

Scope

• cosh (copilot-shell)
• cosh-ng (cosh-ng)
• sec-core (agent-sec-core)
• skill (os-skills)
• sight (agentsight)
• tokenless (tokenless)
• ckpt (ws-ckpt)
• memory (agent-memory)
• anolisa (anolisa-cli)
• skillfs (SkillFS)
• Multiple / Project-wide

Checklist

• I have read the Contributing Guide
• My code follows the project's code style
• I have added tests that prove my fix is effective or that my feature works
• I have updated the documentation accordingly
• For cosh-ng: cargo clippy --all-targets -- -D warnings and cargo fmt --check pass

Testing

• Verified bash syntax with bash -n
• Verified $? preservation through _cosh_suspend_extdebug (exit code 42 → preserved correctly)
• cargo check -p cosh-shell passes

Additional Notes

The return 1 (intercept) path in preexec cannot disable extdebug before
returning — bash checks extdebug AFTER the trap returns to decide whether
to honor the skip. The prepended _cosh_suspend_extdebug in
PROMPT_COMMAND covers this case: after an intercept, the next
PROMPT_COMMAND cycle disables extdebug before system scripts run.