Let plugins create nodes with nothing selected; unship the demo plugins

[dovvnloading]

Problem

executePlugin's wire call carried no spawn position, so a node created without a parent had nowhere host-decided to go. That single gap propagated outward:

• plugin_sdk.py rejected requires_parent=False at registration time, stating exactly that reason.
• So plugins.py demanded a selected parent for every plugin.
• So on an empty canvas, no plugin could be created at all - clicking one produced a warning notification and nothing else.

Three user-visible defects followed:

System Prompt required an existing node. A system prompt is authored before the conversation it governs, yet it could only be added by first selecting a node - so the user had to send a message, spending tokens, purely to unlock the action that should have shaped that message.

Double-click created a blank placeholder node. The handler described itself as "the R1 stand-in for the plugin picker / context menu creation paths". Those paths shipped; the stand-in did not go away, and because plugins needed a pre-existing node, it had become the only way to put a first node on an empty canvas - an undiscoverable gesture producing an untitled node.

Three demo plugins shipped to users. hello_node, counter_node and sandboxed_demo sat in the shipped plugins/ root, so MANIFEST.in's recursive-include plugins packaged them into the wheel and every user's picker listed "Hello Node", "Counter Node" and "Sandboxed Env Probe".

Change

Creation

• The picker sends the canvas viewport's center with every executePlugin call, surfaced to factories and handlers as PluginRunContext.spawn_x/spawn_y. SceneCanvas registers a provider rather than storing a coordinate, so a pan or zoom since the last render cannot yield a stale position.
• requires_parent=False registers now, for both register_node_kind and register_builtin_plugin. add_plugin_node accepts parent_id=None and leaves such a node unconnected.
• System Prompt is parentless-capable: with nothing selected (or a stale selection) it creates the note unattached, deduping against an existing pending one. SceneDocument.adopt_pending_system_prompt then draws the note -> root edge when the first parentless chat node appears - the exact shape _resolve_branch_system_prompt looks for at send time. Without that step the note would be silently inert.
• The double-click create gesture is removed, and zoomOnDoubleClick returns to React Flow's default, which the gesture had displaced.

Demo plugins

• The three move to backend/tests/fixture_plugins/. They remain real plugins exercised through real discovery - the save/reload round-trip and the out-of-process worker isolation tests depend on that - just from a root that ships to nobody. The shipped root is now exactly the seven first-party plugins.

Test plan

• New backend coverage: a pending note is created at the reported spawn point and left unattached; a second call reuses it; an unknown/stale parent id is treated as no selection rather than a blocking warning; a new branch root adopts the pending note and the note -> root edge exists.
• requires_parent=False now asserts a real registration and a requires_parent is False spec, replacing the test that asserted it raised; a sibling test pins the True default.
• New test_no_demo_plugin_ships_in_the_picker pins that none of the three can reappear in the shipped picker. The "More Plugins" catch-all behavior is proven against the fixture root instead.
• test_executeplugin_rejects_wrong_arity_before_running_the_handler updated for the new 4-argument signature.
• Frontend: the picker's spawn point is asserted for the selected, unselected and provider-registered cases, plus one test proving the provider is consulted at click time rather than at render time.
• Full suites on a clean checkout of this branch: 3095 backend passed, 19 skipped; npm run check (schema drift, typecheck, lint, vitest, build, bundle size) green; ruff clean.