docs: README sells four backends and cross-vendor edges, GIF hero

[scgopi]

What changed

• First paragraph names the four backends a loop can run on (Claude Code, GitHub Copilot CLI, Codex, OpenCode) and says an edge never asks which agent is on either end.
• Install asks for at least one of claude, copilot, codex, opencode on PATH instead of Claude Code alone.
• Using it says where the agent is picked in the app (the New loop sheet, default from Settings ▸ New loops use), names --backend claudeCode | copilotCLI | codex | openCode once, and notes that a loop creating children without the flag hands them its own backend.
• How it works stops claiming the daemon schedules nothing: Codex and OpenCode have no /loop skill, so a time-based loop on them takes a daemon heartbeat.
• Hero is now docs/assets/graphcode-demo-hero.gif (1,290,351 bytes): 17.4 s of graphcode-demo.mp4 from 26 s, 12 fps, 960 px wide, 64 colours, gifsicle --lossy=80. It shows jumping between child sessions, the canvas fan-out, and a new loop created with its own agent and done check.

Checked against the code

Claim	Evidence
Four backends, all launchable	CLISessionBackendKind has exactly four cases; isSpiked is true for all
Executable names	BackendCommand.executableName: claude, copilot, codex, opencode
Three edge kinds, all executed	EdgeKind.isExecutable is true for handoff, message, spawn
Edges ignore the backend	No backend comparison anywhere in the handoff/message firing path
--backend values and default	GraphcodeCommand parses the raw enum values; GraphStore fills nil from the creator's backend
Codex/OpenCode heartbeat	BackendCapabilities.supportsDaemonRecurrence and LoopNode heartbeat prompt
Settings label	SettingsView picker titled "New loops use"

Not done

• loop-messaging.mp4 was the first choice for the GIF but has a slow camera zoom on every scene, so every frame is a full redraw: 17.5 s came out at 7.2 MB, and 4.6 MB after denoise and 32 colours. Not recoverable under 1.5 MB at 960 px.
• docs/assets/graphcode-demo.gif (4.8 MB) stays because docs/index.html uses it as the video fallback. screenshots/graph-hero.png is now unreferenced but left in place.
• Docs-only change; no Swift touched, so the Xcode gate was not run.