feat(scheduling): node tags + service placement_tags affinity (#58)

Operator-set node labels (node_agents.tags) gate which nodes a service
may schedule onto: services.placement_tags requires every listed tag.
auto and spread filter eligible agents by tags; tags are re-evaluated
each reconcile so retagging a node re-spreads replicas. Pin + tags and
local + tags rejected; empty array clears the requirement.

Surfaces: --placement-tags on services create/update, nodes update
--tags, MCP args, OpenAPI, node-detail tag editor, service-detail
placement-tags input.

Generated with [Devin](https://devin.ai)

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

ROADMAP2.md

@@ -298,9 +298,18 @@ and wired refs, one click or one CLI call.
   inventory-driven, so remote replicas show up and tear down wherever
   they run. Remote nodes run registry-pulled
   images only — git builds, Traefik domains, and remote sleep are
-  documented ceilings. Remaining: `scheduling_rules` affinity
-  policies, deployment status reported back from agent
-  results, spread weighting by real capacity.
+  documented ceilings. Tag affinity shipped: `node_agents.tags`
+  (operator-set via `PUT /agents/:id` / `nodes update --tags`) +
+  `services.placement_tags` — `auto` and `spread` only consider
+  online schedulable agents carrying every required tag, tags are
+  re-evaluated each reconcile (a tag change re-spreads replicas),
+  unsatisfiable requirements fail with a clear error, pin + tags
+  and local + tags are rejected, empty array clears. Surfaces:
+  `--placement-tags` on services create/update, `--tags` on
+  `nodes update`, MCP args, OpenAPI, node-detail tag editor +
+  service-detail placement-tags input. Remaining: deployment
+  status reported back from agent results, spread weighting by
+  real capacity.
 - [x] **Agent self-upgrade** — `POST /agents/:id/upgrade` enqueues a
   `self_upgrade` command carrying platform + sha256 + server version;
   the agent downloads its replacement from `GET /agents/download/