STATE-WP-0069 T06: add workplan flow entity and prefer open_workplans

Introduce flows/workplan.yaml and /flows/workplan/* routes alongside the
legacy workstream flow. Dashboard, README, and MCP flow docs now prefer
open_workplans with open_workstreams fallback until dual-key retirement.

flows/workplan.yaml

@@ -0,0 +1,59 @@
+id: custodian.workplan.v1
+entity_type: workplan
+workstations:
+  - name: proposed
+    description: Plan exists but needs review against the current repo state.
+    entry_assertions: []
+    exit_assertions: []
+  - name: ready
+    description: Reviewed and ready to execute.
+    entry_assertions: []
+    exit_assertions: []
+  - name: active
+    description: Work is underway.
+    entry_assertions: []
+    exit_assertions:
+      - id: dependencies.all_complete
+        target: dependencies.*.workstation
+        op: all_eq
+        value:
+          - finished
+          - archived
+        description: Dependency workplans have reached a closed state.
+  - name: blocked
+    description: Work is blocked by incomplete dependencies or missing input.
+    entry_assertions:
+      - id: dependencies.any_incomplete
+        target: dependencies.*.workstation
+        op: custom
+        value:
+          - finished
+          - archived
+        description: At least one dependency is not finished or archived.
+    exit_assertions:
+      - id: dependencies.all_complete
+        target: dependencies.*.workstation
+        op: all_eq
+        value:
+          - finished
+          - archived
+        description: All dependency workplans have reached finished or archived.
+  - name: backlog
+    description: Intentionally parked for later.
+    entry_assertions: []
+    exit_assertions: []
+  - name: finished
+    description: Work is complete.
+    entry_assertions:
+      - id: tasks.all_done
+        target: tasks.*.status
+        op: all_eq
+        value:
+          - done
+          - cancel
+        description: All child tasks are done or canceled.
+    exit_assertions: []
+  - name: archived
+    description: Closed work has been moved out of the active set.
+    entry_assertions: []
+    exit_assertions: []