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

ROADMAP.md

@@ -184,6 +184,17 @@ external interfaces. See `docs/architecture/structure.canvas` → *Backend – M
       separate from the cascade rollups, which are pruned on a retention window
       and an experiment must not be. `/runs`, `/runs/{id}`, `/runs/flows/{name}`,
       `/sweep`, `/cancel`, `/metrics` and `/series/compare`
+- [x] A Python SDK, so a research repository is the source of a flow rather than
+      a place code is copied from: `@node` declares a function's ports where the
+      function is, `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 — stamps
+      it with the repository's commit, and retires the workers so the next run
+      imports the code as it is now. `fluksio login|run|runs` and
+      `flow.submit().wait()` are the client half. The decorators return the
+      function untouched, which is the whole point: it is still your code, still
+      callable, and `device="gpu"` on the same decorator is where it runs
 - [x] Streaming outputs: a node that produces values over time is a generator,
       and every `yield` is a dict keyed by output port, published the instant it
       happens; what it returns is its result. A port doing this declares