feat(shell): vim and vix via registry packages, delete .local-cmds

[NathanFlurry]

• Replace the shell's .local-cmds runtime package synthesis (~60 lines) with the @agentos-software/vim and @agentos-software/vix registry packages — vim's runtime tree + VIMRUNTIME now come from the package manifest provides (secure-exec feat: add camel names to meta #248); unbuilt placeholders are skipped instead of projected
• Declare the shell's phantom dependencies (@rivet-dev/agentos, rivetkit) that a strict install exposed
• Point the vim test suites at the registry package's dist/package/bin/vim (env override kept; CI placeholder → skip preserved)
• Bump .github/refs/secure-exec to the vim/vix registry merge

Validated in-VM: vim --version → 9.2 from /opt/agentos/bin/vim, runtime overlay + VIMRUNTIME applied, :wq writes files (a pre-existing main regression currently suppresses insert-mode repaints — tracked separately; file writes verified byte-exact).