fix: use query params for workflow_id in Poimen endpoints

Root cause: ServiceAdapter dispatcher sets X-Upstream-Path to template
strings like /v1/poimen/executions/{id}/stream with literal {id}.
extractWorkflowID() returned '{id}' instead of the actual workflow ID.

Fix:
- All handlers read workflow_id from ?workflow_id=xxx query param
- Remove path param extraction (extractWorkflowID)
- Update spec paths: remove {id} segments, use flat resource paths
  /v1/poimen/executions/detail?workflow_id=xxx
  /v1/poimen/executions/stream?workflow_id=xxx
  /v1/poimen/executions/history?workflow_id=xxx
  /v1/poimen/executions/signal?workflow_id=xxx
- Exact path matching in ServeHTTP (no prefix/suffix matching)

API contract:
  GET  /v1/poimen/executions                         — list all
  GET  /v1/poimen/executions/detail?workflow_id=xxx   — describe one
  GET  /v1/poimen/executions/history?workflow_id=xxx  — event history
  GET  /v1/poimen/executions/stream?workflow_id=xxx   — SSE events
  POST /v1/poimen/executions/signal?workflow_id=xxx   — signal
  DELETE /v1/poimen/executions/detail?workflow_id=xxx  — terminate

internal/serviceadapter/poimen_adapter.go

@@ -53,17 +53,20 @@ func GetPoimenSpec() *Spec {
 					},
 				},
 			},
+			// Note: workflow_id is passed as ?workflow_id=xxx query param,
+			// not as a path segment. The ServiceAdapter dispatcher does not
+			// substitute path parameters.
 			{
 				Name: "execution-detail",
 				Methods: []Method{
 					{
 						Verb:         "GET",
-						UpstreamPath: "/v1/poimen/executions/{id}",
+						UpstreamPath: "/v1/poimen/executions/detail",
 						Auth:         &Auth{Required: true, Capability: "workflow:read"},
 					},
 					{
 						Verb:         "DELETE",
-						UpstreamPath: "/v1/poimen/executions/{id}",
+						UpstreamPath: "/v1/poimen/executions/detail",
 						Auth:         &Auth{Required: true, Capability: "workflow:execute"},
 					},
 				},
@@ -72,7 +75,7 @@ func GetPoimenSpec() *Spec {
 				Name: "execution-history",
 				Methods: []Method{{
 					Verb:         "GET",
-					UpstreamPath: "/v1/poimen/executions/{id}/history",
+					UpstreamPath: "/v1/poimen/executions/history",
 					Auth:         &Auth{Required: true, Capability: "workflow:read"},
 				}},
 			},
@@ -80,7 +83,7 @@ func GetPoimenSpec() *Spec {
 				Name: "execution-stream",
 				Methods: []Method{{
 					Verb:         "GET",
-					UpstreamPath: "/v1/poimen/executions/{id}/stream",
+					UpstreamPath: "/v1/poimen/executions/stream",
 					Auth:         &Auth{Required: true, Capability: "workflow:read"},
 				}},
 			},
@@ -88,7 +91,7 @@ func GetPoimenSpec() *Spec {
 				Name: "execution-signal",
 				Methods: []Method{{
 					Verb:         "POST",
-					UpstreamPath: "/v1/poimen/executions/{id}/signal",
+					UpstreamPath: "/v1/poimen/executions/signal",
 					Auth:         &Auth{Required: true, Capability: "workflow:execute"},
 				}},
 			},