Add a Python SDK: flows declared in your own repository

A data scientist keeps their code where it is and decorates it: `@node`
declares a function's ports beside the function, `Flow(name, nodes=[...])`
says which of them make a flow, and `use(fn, wire=..., **settings)` rebinds
one for a single flow. `fluksio sync` uploads the document plus a generated
import shim per node, so the store still holds a complete, runnable,
git-versioned definition while the code it imports stays theirs.

`fluksio login|run|runs` and `flow.submit().wait()` are the client half, over
the run endpoints that already existed. Runs record the user repository's
commit beside the store's, so "what code produced this number" is answerable
on the side that now holds the code.

- `fluksio/sdk/`: ports, decorators, the flow builder and its checks, the shim
  generator, an HTTP client and sync. Standard library only at import, so
  `from fluksio import node` in a training script pulls in no engine.
- `FlowDef.origin` marks a flow code-defined; `Run.origin_commit` carries the
  repository's commit; `POST /modules/refresh` retires the workers without an
  install, which every sync calls — a worker holds the imported package in
  memory, so an edit to it is invisible until the process goes.
- The canvas shows a generated body read-only and names the repository to edit
  instead; a body edited there stops the next sync rather than being discarded.
- The worker's reporter carries inert `Port`, `node`, `use` and `Flow`, since
  the shim imports a module whose first line declares them.
- `examples/myresearch` is the worked example, `make sync-example` uploads it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012ue1tkFWB1bcGy3aWhCKpU

docs/code/nodes.md

@@ -204,6 +204,15 @@ def process(lr, epochs):
 That is the whole of it. The node is three lines, `myresearch` can be as many
 modules as it likes, and nothing was copied.
 
+!!! tip "You can have those three lines written for you"
+
+    Decorate `fit` with `@node(...)` where it is defined, say which nodes make
+    a flow with `Flow(...)`, and `fluksio sync` generates the body above —
+    along with the flow document, so there is nothing to PUT by hand. The
+    declaration lives beside the function it describes and is checked against
+    its signature. See
+    [Getting started: data science](../getting-started/data-science.md).
+
 !!! warning "Editable, but not live"
 
     `-e` means edits reach the venv without reinstalling — but a node's process