profiles: extract shared sandbox profile for enzo + kusanagi

Move the yokai agent-sandbox account (user, ikai group, /srv/git drop dir,
scoped sudo de-escalation, and the local-clones workflow docs) out of
machines/enzo into a shared profiles/sandbox.nix that both machines import.

The only per-machine value is the human owner allowed to drop into the
sandbox, exposed as a `sandbox.owner` string option so the profile stays
username-agnostic (enzo: hwebs, kusanagi: henz). kusanagi gains the sandbox
it didn't have before; enzo's inline block collapses to one setting.

Assisted-by: claude-code:claude-opus-4-8

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