wit: land the host-intent decouple master gate for an acyclic split

[mfw78]

This epic lands the host-intent decouple as the master gate and folds the whole reshape to a green, CI-verifiable tip while nothing is pinned yet.

Goal

Land the free in-monorepo reshape: the host-intent decouple, where the host event carries opaque status bytes, as the master gate, plus the single oracle-validated git-filter-repo fold to a green tip, so the acyclic split becomes CI-verifiable while nothing is pinned.

Scope

The host world currently imports the intent package, which makes an acyclic three-repo split physically impossible; this epic breaks that import first, then reshapes and re-versions the contract as one train-wide fold rather than as per-car edits. Alongside the decouple it stands up the CI invariant that keeps nexum-runtime venue-agnostic, keeps the egress guard advisory-only for now, and closes the busy-loop DoS on guard-deny. The work culminates in a single green linear tip that is the precondition for any later repository carve.

Milestone: M1: Videre contract reshape and host-intent decoupling.

[mfw78]

Done. The M1 host-intent-decouple master gate is fully landed on dev/m1: opaque-status host decouple (#360, via #426), the videre reshape and re-version (#368/#370/#373, via #428/#429/#431), the advisory venue-agnostic CI invariant (#362, via #432), the advisory egress guard (#363, via #433), and the guard-deny busy-loop DoS fix (#364, via #435). The green linear tip was reached and carved at M5. The residual dev/m1 merge bookkeeping for later milestones is tracked by #367.