Reject shell operators in read-only exec commands

[OnlyYu1996]

Summary

• reject read-only exec commands that contain shell control operators such as redirects, pipes, command chaining, background execution, command substitution, or newlines
• keep simple read-only commands and quoted literal operator text working
• add regression coverage for the redirection/chaining bypass cases
• fix the stale agent command test import needed for current main test builds

Fixes PlatformNetwork/bounty-challenge#53178.

Validation

• cargo fmt --package cortex-cli --check
• cargo test --package cortex-cli exec_cmd::autonomy::tests -- --nocapture
• cargo check --package cortex-cli --lib
• git diff --check

[greptile-apps]

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