feat(runtime): refuse an egress policy the host tier cannot enforce

[rigel-mintaka]

This PR is part of a stack containing 10 PRs:

1. main
2. feat(runtime): add the host-process WorkloadRuntime backend (RIG-3512) #1120
3. feat(runner): derive the agent uid from the Runner's euid on the host backend (RIG-3512) #1125
4. feat(runner): deliver the agent socket and config by path on the host backend (RIG-3512) #1135
5. feat(runtime): distinguish a configured egress policy from an unset one #1142
6. "feat(runtime): refuse an egress policy the host tier cannot enforce" (this PR)
7. feat(runner): carry no egress policy on a backend that cannot enforce one #1145
8. feat(runner): report the runtime tier and egress posture per session #1148
9. feat(cli): show the runtime tier and egress posture per session #1153
10. feat(ui): mark each agent's runtime tier and egress posture #1154
11. feat(runner): declare the runtime tier and egress posture at enrollment #1156

A host child shares the host's network namespace, so the nftables arm the
container tiers run has no boundary to attach to. The host backend now reports
that through an egressUnenforcer marker, and provision refuses any configured
policy instead of dropping it silently — a caller must never believe egress was
constrained when nothing constrained it.

The marker is deliberately separate from inGuestEgressArmer: that one means
someone armed the firewall, this one that nobody did and nobody can.
AgentRuntime.EgressPosture reports which, so a session surface can show an
uncontained launch as uncontained.

The refusal keys on the policy's presence, not a non-empty allowlist: an empty
allowlist is pure default-deny, the strictest posture, so keying on length
would reject a looser policy and silently discard the tightest one.

[linear-code]

RIG-3512

[github-actions]

Compass engineering docs preview: https://compass-service-owner-rig-35-tzgh.compass-eng-docs.pages.dev

Deployed from compass-service-owner/rig-3512-egress-unenforced at df90e31.

[trunk-io]

This pull request was merged into main as part of stacked PR 1156.