Propagate correlation_id/causation_id through execution events, add API version header

Execution events (started/failed/succeeded) never carried the
correlation/causation IDs from the ExecuteRequest, even though the
fields existed on Execution. Added CausationID to ExecuteRequest and
Execution, and split emitEvent into a correlated variant used
throughout the execution lifecycle. handleExecute falls back to
X-Correlation-ID/X-Causation-ID headers when the body omits them.
Also add X-Hexis-Version negotiation on /api/v1/*, matching Nexus.

Part of Vikunja #273.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018ghELqYhZNLub2TXGMazqA

pkg/client/client.go

@@ -10,6 +10,35 @@ import (
 	"time"
 )
 
+// APIVersion is sent as X-Hexis-Version on every request so the server can
+// negotiate/reject unsupported client versions.
+const APIVersion = "v1"
+
+type correlationKey struct{}
+type causationKey struct{}
+
+// WithCorrelationID returns a context that carries a correlation ID to be
+// sent as X-Correlation-ID on every Hexis request made with it.
+func WithCorrelationID(ctx context.Context, id string) context.Context {
+	return context.WithValue(ctx, correlationKey{}, id)
+}
+
+// WithCausationID returns a context that carries a causation ID to be sent
+// as X-Causation-ID on every Hexis request made with it.
+func WithCausationID(ctx context.Context, id string) context.Context {
+	return context.WithValue(ctx, causationKey{}, id)
+}
+
+func correlationIDFrom(ctx context.Context) string {
+	id, _ := ctx.Value(correlationKey{}).(string)
+	return id
+}
+
+func causationIDFrom(ctx context.Context) string {
+	id, _ := ctx.Value(causationKey{}).(string)
+	return id
+}
+
 type Client struct {
 	baseURL    string
 	httpClient *http.Client
@@ -37,6 +66,13 @@ func (c *Client) do(ctx context.Context, method, path string, body, result any)
 		return fmt.Errorf("create request: %w", err)
 	}
 	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("X-Hexis-Version", APIVersion)
+	if id := correlationIDFrom(ctx); id != "" {
+		req.Header.Set("X-Correlation-ID", id)
+	}
+	if id := causationIDFrom(ctx); id != "" {
+		req.Header.Set("X-Causation-ID", id)
+	}
 
 	resp, err := c.httpClient.Do(req)
 	if err != nil {
@@ -83,6 +119,7 @@ type Execution struct {
 	Error           string            `json:"error,omitempty"`
 	RequestedBy     map[string]string `json:"requested_by,omitempty"`
 	CorrelationID   string            `json:"correlation_id,omitempty"`
+	CausationID     string            `json:"causation_id,omitempty"`
 	IdempotencyKey  string            `json:"idempotency_key,omitempty"`
 }
 
@@ -94,6 +131,7 @@ type ExecuteRequest struct {
 	Origin          map[string]string `json:"origin,omitempty"`
 	IdempotencyKey  string            `json:"idempotency_key,omitempty"`
 	CorrelationID   string            `json:"correlation_id,omitempty"`
+	CausationID     string            `json:"causation_id,omitempty"`
 }
 
 type CreateCapabilityRequest struct {