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.

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

docs/guides/CLI.md

@@ -152,6 +152,15 @@ containr services update <id> --replicas 4 --spread    # replica i → node[i %
 containr services update <id> --spread=false           # back to single placement
 ```
 
+Constrain `auto`/`spread` placement to nodes carrying operator tags —
+`nodes update` labels a node, `--placement-tags` requires every listed tag
+(mutually exclusive with a pin; an empty value clears the requirement):
+
+```bash
+containr nodes update <id> --tags gpu,eu-west
+containr services update <id> --placement-tags gpu     # only gpu nodes are eligible
+```
+
 Remote nodes run registry-pulled images only — git builds, domains
 (Traefik lives on the host), and sleep detection stay local. Start,
 stop, restart, redeploy, and delete all fan out to the pinned node's