Harden assimilate.sh: arch-aware delta, quoted $HOME, perms

[akan72]

Summary

Branch 6: harden assimilate.sh and bash_profile.

• Arch-aware delta install: detect arch with uname -m. Apple Silicon uses the pinned aarch64-apple-darwin tarball; Intel Macs fall back to brew install git-delta.
• Quote $HOME in bash_profile: if [ -f "$HOME/.bashrc" ]
• chmod 600 ~/.gitconfig and chmod 700 ~/.ssh (when present) after symlinking

Note on arch detection: delta v0.19.2 ships no x86_64-apple-darwin build (only aarch64-apple-darwin for macOS). So the literal "select x86_64-apple-darwin" would 404. There's no pinned tarball to checksum-verify on Intel, so the Intel path falls back to Homebrew instead.

Why & alternatives (from explanations.txt — Branch 6)

Change	Why	Alternatives
Arch detection for delta	aarch64-apple-darwin was hardcoded — breaks on Intel Macs.	Drop the pinned tarball entirely and use brew install git-delta (the pinned approach exists due to brew bottle ABI drift).
Quote $HOME in bash_profile	[ -f $HOME/.bashrc ] breaks on paths with spaces. Defensive.	Skip — username has no spaces.
chmod 600 on sensitive symlinks	Defense against world-readable secrets.	Skip — home dir is probably mode 700.

Note: Branches 6 and 11 both touch assimilate.sh; expect a trivial conflict to resolve when merging the second one.