feat(runner): report the runtime tier and egress posture per session

[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 #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" (this PR)
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 green launch said nothing about which boundary an agent actually got, and the
host tier has none. AgentSessionStatus now carries both the tier and the egress
posture, stamped by the Runner because it is the component that resolved the
backend.

Posture is carried separately rather than derived from the tier: enforcement is
a property of the backend, so a client inferring one from the other would render
a stale answer the moment that changes. Both fields are backward-compatible
appends, and an unrecognized value maps to UNSPECIFIED rather than a plausible
default — on a security surface, saying nothing beats guessing.

Tier identity rides a capability probe rather than widening the frozen
WorkloadRuntime interface, matching the existing egress markers.

[linear-code]

RIG-3512

[github-actions]

Compass engineering docs preview: https://compass-service-owner-rig-35-3bn7.compass-eng-docs.pages.dev

Deployed from compass-service-owner/rig-3512-egress-posture-wire at b6f3d69.

[trunk-io]

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