fix(cli): prevent symlink attacks during instruction writes

[tachyon-beep]

Motivation

• The instruction installer used a predictable PID-based sibling temp path and fs::write, which could follow a pre-planted symlink and allow attacker-influenced arbitrary overwrites during atomic_write.
• The change hardens the atomic-write path to avoid following attacker-controlled symlinks while preserving existing behaviour like same-filesystem atomic rename and permission preservation.

Description

• Replace the deterministic temp-path + fs::write pattern with tempfile::Builder::tempfile_in(parent) and an exclusive NamedTempFile, then write_all and persist to atomically install the file in the target directory.
• Preserve the existing guards: refuse empty payloads and retain permission copying via preserve_mode, and keep the write/rename as a same-filesystem atomic operation.
• Promote tempfile to a runtime dependency and add use std::io::Write; for write_all.
• Add a regression test atomic_write_does_not_follow_predictable_temp_symlink that plants the legacy predictable temp symlink and verifies the victim is not overwritten while the intended target is updated.

Testing

• Ran cargo fmt --check, which succeeded.
• Ran cargo test -p loomweave-cli instructions::tests::atomic_write, and the targeted tests passed (atomic_write_refuses_empty_content, atomic_write_preserves_mode, atomic_write_does_not_follow_predictable_temp_symlink).
• Ran cargo clippy -p loomweave-cli --all-targets -- -D warnings, which completed without warnings.
• Verified repository state with git diff --check and git status --short after committing the fix.

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[Copilot]

Pull request overview

This PR hardens loomweave-cli’s instruction installer against symlink-based arbitrary overwrite attacks by replacing a predictable, PID-based temp file write with a securely-created temporary file that cannot be pre-symlinked by an attacker, while keeping the same-directory atomic rename behavior and permission preservation.

Changes:

• Replace deterministic sibling temp-path + fs::write with tempfile::NamedTempFile created via tempfile_in(parent), followed by write_all and persist.
• Preserve Unix permission-copy behavior via preserve_mode before the final atomic persist/rename.
• Add a Unix regression test that plants the legacy predictable temp-path symlink and asserts the “victim” file is not overwritten.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
crates/loomweave-cli/src/instructions.rs	Uses NamedTempFile for safe atomic writes and adds a regression test for the predictable-temp symlink attack.
crates/loomweave-cli/Cargo.toml	Promotes tempfile to a runtime dependency to support the hardened write path.

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[tachyon-beep]

Superseded by the rebase onto release/1.6.0, which also widens the fix to the other five staging sites of the same class and keeps umask-derived file modes.