docs(arch): draft alknet-docker architecture specs (ADRs 058-063, OQs 048-051)

Greenfield architecture spec set for the alknet-docker crate — a thin,
single-host bollard wrapper exposing docker container/image operations
as call-protocol ops on the shared alknet/call ALPN, plus a
DockerTtyBackend (impl TtyBackend) behind a tty feature for interactive
terminal sessions into containers over alknet/tty.

Six ADRs:
- 058: docker ops on alknet/call (no separate ALPN; raw-carriage handoff
  dissolved by alknet-tty extraction — interactive attach moved to
  alknet/tty via DockerTtyBackend, no carriage field on call.requested)
- 059: bollard 0.21 (verified current on crates.io) + feature selection
  (http+pipe+time; no ssl/ssh/websocket/buildkit)
- 060: container resource model (ADR-050 application) — alknet.managed/
  alknet.owner labels, list owned_only flag, hosted-services operator
  role via static-resource fallback, handler-driven revoke with
  autonomous-death tolerance, resource-action vocabulary
- 061: DockerTtyBackend in alknet-docker behind tty feature (attach vs
  exec mode; POC drive_attach_raw as reference)
- 062: Docker client + OwnershipStore injection via closure capture
  (not Capabilities, not OperationContext — matches from_openapi pattern)
- 063: exit code on terminal call.responded for non-interactive exec
  (call.completed stays empty, ADR-012 unchanged)

Four spec docs (crates/docker/): README, overview, docker-operations,
docker-tty-backend. Four deferred-scope OQs (048-051): network/volume
ops, buildkit, system events subscription, create options surface.

Updates the tty-backend.md Backend implementations table (DockerTtyBackend
row now specced) and the architecture README (doc table, ADR table,
current-state paragraph, OQ count).

Grounded in the alknet-docker POC (docs/research/alknet-docker/poc-summary.md)
which validated the hard parts; the remaining lifecycle ops are mechanical
bollard wrapping. Reviewed by architecture-reviewer subagent; criticals
(the docker_client injection model conflicting with the Capabilities
contract) resolved via ADR-062/063 before commit.

docs/architecture/questions/050-docker-system-events-subscription.md

@@ -0,0 +1,34 @@
+# OQ-50: Docker System Events Subscription
+
+- **Origin**:
+  [crates/docker/docker-operations.md](crates/docker/docker-operations.md)
+  §"Out of scope for v1";
+  [ADR-060](decisions/060-container-resource-model-and-label-namespace.md)
+  §4 (autonomous-death tolerance).
+- **Status**: deferred(scope)
+- **Door type**: Two-way
+- **Priority**: low
+- **Blocked on**: a concrete use case for prompt stale-ownership
+  cleanup. The base model (ADR-060 §4) tolerates stale ownership
+  entries from autonomous container death (a `--rm` exit, external
+  `docker rm`, daemon restart) — they're inert (a reused container ID
+  gets a fresh `record` on its next `create`). A reaper that
+  subscribes to docker daemon events would clean them promptly, but
+  the promptness gain is marginal for the current use cases.
+- **Resolution**: Not yet decidable. bollard's `events()`
+  (system.rs:128) returns a `Stream<SystemEventsMessage>` of daemon
+  events (container start/stop/die/destroy, image pull, etc.). A
+  `docker/system/events` `Subscription` operation would surface these
+  as `call.responded` frames; the ownership store could subscribe
+  internally to revoke on `destroy` events. The deferral is scope:
+  the base model works without it; the events subscription is a
+  refinement for when prompt cleanup matters (e.g., a high-churn
+  coordinator that spawns/removes many containers and wants the
+  ownership store to stay tight). Adding it is additive (a new
+  operation + an internal store subscription) and does not break the
+  existing surface.
+- **Cross-references**:
+  [ADR-060](decisions/060-container-resource-model-and-label-namespace.md)
+  §4 (teardown coupling — stale-entry tolerance is the base model
+  this subscription would refine),
+  [crates/docker/docker-operations.md](crates/docker/docker-operations.md)