feat: read agent_id from ctx (SDK now plumbs it)

Plexum-sdk-go now unpacks `_meta.agent_id` from host's MCP tools/call
frames into the ctx. Plugin reads it via plugin.AgentIDFromContext
and uses it for backend api-key resolution (agentKeys map lookup +
bearer token) and HF on_call pre-check.

config.defaultAgentID demoted from "v1 stop-gap for the missing SDK
plumbing" to "fallback for operator-driven plugin-call". Normal turn
loops carry the real caller id through ctx now; defaultAgentID only
fires on host-driven dispatch paths.

Also: log every CallTool dispatch with {tool, agent_id} at info level
so operators can see which agent is hitting which tool without
debug-level chatter.

E2E in sim: PLEXUM_ECHO_FORCE_TOOL_USE drives echo provider →
dialectic_list_topics dispatched via host → plugin sees
agent_id=test-agent in ctx + logs it correctly.

cmd/plexum-dialectic-plugin/main.go

@@ -56,9 +56,14 @@ func (p *dialecticPlugin) Init(ctx context.Context, host sdkplugin.HostAPI) erro
 		Config: p.cfg,
 		Host:   host,
 		AgentIDFromCtx: func(ctx context.Context) string {
-			// v1: SDK doesn't surface the calling agent on tool ctx.
-			// Fall back to the per-claw default. Multi-agent claws will
-			// need SDK ctx plumbing — tracked upstream.
+			// Host attaches the caller agent id via tools/call
+			// `_meta.agent_id`; SDK unpacks it into ctx.
+			if id := sdkplugin.AgentIDFromContext(ctx); id != "" {
+				return id
+			}
+			// Fallback for host paths that don't carry an agent
+			// (CLI plugin-call against this plugin from an operator
+			// debugging the deployment). Empty when not set.
 			return p.cfg.DefaultAgentID
 		},
 	}
@@ -66,6 +71,10 @@ func (p *dialecticPlugin) Init(ctx context.Context, host sdkplugin.HostAPI) erro
 }
 
 func (p *dialecticPlugin) CallTool(ctx context.Context, name string, input json.RawMessage) (sdkplugin.ToolResult, error) {
+	p.host.Log("info", "tools/call dispatched", map[string]any{
+		"tool":     name,
+		"agent_id": sdkplugin.AgentIDFromContext(ctx),
+	})
 	return tools.Dispatch(ctx, p.deps, name, input)
 }