Refresh the README: new screenshots, the Builder, and four stale claims

[dovvnloading]

Problem

The README had not been updated in months. Its screenshots predated the app bar redesign, the View panel rebuild, the canvas connection rewrite, and the Builder entirely. Several statements had also become false, and a substantial amount of shipped functionality had never reached it at all.

Change

Screenshots. Four new ones, committed under assets/screenshots/ rather than linked to GitHub upload URLs so they live with the code that produced them: the branching canvas (hero), a finished Builder run showing its checklist/budgets/activity log beside the nodes it created, the Builder launch dialog, and a code-to-chart workflow. Each was captured from the real app rendering real scene state — the demo content is authored, the rendering is not.

Corrected claims, each verified against the code before changing:

Claim	Reality
Provider mode "isn't wired up in the UI yet"	Settings has fired the real setProviderMode intent since ADR-006/ADR-012
"no way to attach or ingest a file from the interface"	The composer has had real staged attachments since R8a
No "browser-driven UI testing"	A Playwright suite runs as its own CI job (ADR-015)
CI is two jobs	It is four: Python checks, frontend checks, E2E, and a wheel build check

Added. A section on the Builder, plus feature and node-kind coverage for work that had landed without ever reaching the README: the knowledge base and global search, workspaces, undo of a whole agent run, per-branch model pinning, charts, themes, the plugin SDK with capability grants, MCP servers, diagnostics, and the token/cost counter. The node-kind list went from 5 to the real 15, and the plugins/ package layout ADR-014 introduced is now documented.

Test plan

• Every image path resolves to a committed file
• Every table-of-contents anchor resolves to exactly one heading
• Each corrected claim checked against the registered intent or CI job that disproves the old text
• Each newly claimed feature checked against its real registered intent (knowledge/search, globalSearch/search, scene/undoRun, scene/setModelOverride, app-settings/setTheme, app-settings/setMcpServers, app-chat-library/createWorkspace, app-chat-library/exportWorkspace, app-plugins/setPluginGrant, diagnostics/exportDiagnosticBundle)
• Demo figures in the screenshots are internally consistent (the per-job deltas sum to the stated total; the top-3 share matches the listed durations)

Docs and images only — no application code changes.