refactor(compute): negotiate external driver behavior

[drew]

Summary

Make post-initialization compute-driver behavior depend on the public gRPC contract instead of concrete in-tree driver names. This stacked follow-up lets canonical driver names be supplied over UDS and negotiates whether the gateway owns graceful-shutdown/startup lifecycle reconciliation.

Related Issue

Closes #2417

Stacked on #2743.

Changes

• add additive compute-driver feature negotiation for gateway-managed lifecycle and native process identity
• advertise GATEWAY_MANAGED_LIFECYCLE from Docker, Podman, and VM; Kubernetes and legacy external drivers remain opted out
• use that capability for both shutdown StopSandbox and startup StartSandbox sweeps
• keep unspecified process-identity normalization as a separate capability because it occurs before the driver request
• structurally validate driver-requested callback listeners independently of driver name
• allow canonical built-in names with --compute-driver-socket, with the explicit endpoint taking precedence
• prove shutdown, startup, lifecycle, and policy parity through a fake external UDS driver using the canonical docker name
• document the supported extension boundary in architecture notes and agent diagnostics; no public docs/ changes are included in this PR

Testing

• mise run pre-commit
• focused core capability, shutdown/startup reconciliation, and external UDS lifecycle tests
• OPENSHELL_E2E_DOCKER_TEST=gateway_start mise run e2e:docker
• mise run test: all completed suites passed except the unrelated gateway_completer_returns_empty_when_no_config, which reproduces in isolation because this host has registered system gateway configuration

Checklist

• Changes are scoped to the accepted issue
• Tests cover capability-driven behavior and the external UDS path
• Architecture notes and agent diagnostics are updated
• Public-facing documentation changes remain in refactor(compute): unify gateway restart reconciliation #2743, not this follow-up

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[elezar]

Features are essentially binary flags, correct. Do we see the need for something richer?