feat(runtime): distinguish a configured egress policy from an unset one

[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" (this PR)
6. feat(runtime): refuse an egress policy the host tier cannot enforce #1143
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

An empty allowlist is the strictest posture — pure default-deny — while a
zero-value policy means egress was never configured. The two were
indistinguishable, so a tier that cannot enforce egress had no way to refuse a
policy without also rejecting the tightest one.

AllowEgress now marks the policy configured, and Configured() exposes it.
Hosts() and NftScript() are untouched, so container arming is unchanged.

[linear-code]

RIG-3512

[github-actions]

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

Deployed from compass-service-owner/rig-3512-egress-configured at ab53b80.

[trunk-io]

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