Add the connector contract, reusable nodes and per-port intervals

Connectors are the device-facing node class third parties write, so the
surface they build against is versioned and documented: ConnectorNode carries
a declared contract version, a polling loop that publishes only what changed
and reports health around it, and parameters whose credential fields are
marked x-secret so the editor offers the secrets store instead of a text box.
They are found through the fluksio.node_types entry point group, with the
package's own metadata as the manifest. docs/connectors/ has the contract and
the authoring guide; connector-skeleton/ is a working one to copy.

The controller no longer knows what any node type is: start, stop and
report_health are protocol methods on Node, and the built-ins were migrated to
them first, so the hooks a connector implements are the ones the engine has
been driving all along.

Marking a node reusable moves its source to _lib/ and points the node at it by
name. Other flows instantiate it with their own ports and settings, one fix
reaches all of them, and a shared source still in use cannot be deleted.

Ports gained an interval: an output publishes, and an input wakes its node, at
most every n seconds. State keeps the latest value, so only the delivery is
skipped, and pressing Run is never throttled.

Also fixes autosave sending no version on its first save of a session, which
made every flow saved more than once conflict with itself.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/components/Flow/CommandPalette.tsx

@@ -1,3 +1,4 @@
+import { useQuery } from "@tanstack/react-query"
 import { useNavigate } from "@tanstack/react-router"
 import { useEffect } from "react"
 
@@ -10,6 +11,7 @@ import {
   CommandItem,
   CommandList,
 } from "@/components/ui/command"
+import { libraryQueryOptions } from "./queries"
 
 /**
  * ⌘K: add a node, jump to another flow, or run the current one, without
@@ -21,6 +23,7 @@ export function CommandPalette({
   nodeTypes,
   flows,
   onAddNode,
+  onAddSharedNode,
   onRun,
 }: {
   open: boolean
@@ -28,9 +31,14 @@ export function CommandPalette({
   nodeTypes: NodeTypeInfo[]
   flows: FlowSummary[]
   onAddNode: (type: string) => void
+  onAddSharedNode: (libName: string) => void
   onRun: () => void
 }) {
   const navigate = useNavigate()
+  const { data: libraryData } = useQuery({
+    ...libraryQueryOptions(),
+    enabled: open,
+  })
 
   useEffect(() => {
     const onKey = (event: KeyboardEvent) => {
@@ -48,6 +56,8 @@ export function CommandPalette({
   // dialog outright is deterministic; the palette does not need to fade out.
   if (!open) return null
 
+  const library = libraryData ?? []
+
   const close = (action: () => void) => {
     onOpenChange(false)
     action()
@@ -81,6 +91,28 @@ export function CommandPalette({
           ))}
         </CommandGroup>
 
+        {library.length > 0 ? (
+          <CommandGroup heading="Reusable nodes">
+            {library.map((entry) => (
+              <CommandItem
+                key={entry.name}
+                value={`shared ${entry.name}`}
+                onSelect={() => close(() => onAddSharedNode(entry.name))}
+              >
+                <span className="flex flex-col">
+                  <span className="font-mono">{entry.name}</span>
+                  <span className="text-xs text-muted-foreground">
+                    {(entry.used_by ?? []).length === 1
+                      ? "Used once"
+                      : `Used ${(entry.used_by ?? []).length} times`}
+                    , code shared with every flow using it
+                  </span>
+                </span>
+              </CommandItem>
+            ))}
+          </CommandGroup>
+        ) : null}
+
         {flows.length > 0 ? (
           <CommandGroup heading="Flows">
             {flows.map((flow) => (