runtime: ship an advisory-only guard posture and document the non-enforcing checkpoint

[mfw78]

Keep the egress guard advisory-only for this milestone: keep AllowAllGuard as the default, feature-gate the pool import, and document the checkpoint as not yet enforcing.

Why

The egress guard is AllowAllGuard, a no-op (pool_router.rs lines 104-110), and the real guard is deferred wholly to the egress-guard epic. This milestone must not advertise a boundary it does not enforce. Part of milestone M1: Videre contract reshape and host-intent decoupling. See docs/design/videre-split-plan.md and docs/design/issue-milestone-plan.md.

Scope

• Keep AllowAllGuard as the default guard.
• Feature-gate the nexum:intent/pool import so the advertised derive to guard to submit checkpoint is not shipped as enforcing in the default build.
• Document at the router seam and in venue docs that the checkpoint is advisory-only and not yet enforcing, with a forward pointer to the egress-guard epic.

Done when

• AllowAll is kept as the default.
• The pool import is feature-gated.
• The router seam and venue docs mark the checkpoint advisory-only for this milestone with a link to the guard epic.

[mfw78]

Landed via #433 (squash-merged into dev/m1). Closing manually since the merge target is not the default branch.