Replay Portable AI Kit scaffold on current main

[mdheller]

Summary

Replays the Portable AI Kit scaffold from draft #16 onto current main without overwriting newer command routers.

Changes

• Add portable-ai route to the current bin/sourceosctl.
• Add Portable AI command parser and minimal helper modules.
• Add standalone bin/sourceos-portable-ai entrypoint.
• Add repo-local Homebrew formula scaffold.
• Add packaging validator and install/smoke guide.
• Wire Portable AI and packaging validation into the current Makefile.

Validation target

make validate
python3 scripts/validate_packaging.py
python3 bin/sourceosctl portable-ai profiles
python3 bin/sourceosctl portable-ai preflight /tmp/SOURCEOS_AI --profile tiny-router
python3 bin/sourceosctl portable-ai prepare /tmp/SOURCEOS_AI --profile tiny-router --dry-run

Capture posture

Supersedes draft #16 after current-base CI passes. The old branch is substantially behind current main, so this replacement preserves current router state.