Restore Linux process dumpability hardening

[noobping]

Motivation

• Fix a security regression where the prctl(PR_SET_DUMPABLE, 0) hardening was gated behind the flatpak feature and therefore not applied in default non-Flatpak Linux builds, allowing same-UID processes to read memory.

Description

• Remove the #[cfg(feature = "flatpak")] gate so disable_process_dumpability() is compiled and its call is executed on all Linux builds, restoring the prctl(PR_SET_DUMPABLE, 0, ...) hardening alongside core-dump limits in src/support/hardening.rs.

Testing

• Ran cargo fmt --check which passed.
• Ran git diff --check which passed.
• Ran cargo check which failed due to a missing system dependency (glib-2.0.pc not found by pkg-config) in the environment, so full build verification was blocked.

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Codex Task