docs(README): comprehensive refresh — landing-page icon (SVG, light/dark) + 8 runtimes + Canvas v4 + Memory v2 + SaaS + channel plugin

The README hadn't been refreshed since the v0 wave. Several major
shipped surfaces weren't called out (Canvas v4 warm-paper theme,
Memory v2 with pgvector, RFC #2967 typed-SSOT A2A response path,
the SaaS control plane, the molecule-mcp-claude-channel plugin we
just shipped via v0.4.0/0.4.1/0.4.2). The runtime list still said
"6" when 8 are in production. The icon was a 1.3 MB PNG with no
light-mode variant.

- New `docs/assets/branding/molecule-icon.svg` matches the landing
  page's `public/favicon.svg` shape (5-spoke molecular graph) but
  carries `prefers-color-scheme` styles so it adapts to GitHub's
  light/dark modes. The PNG stays for back-compat with anything
  that hotlinks it.
- `docs/assets/branding/molecule-logo.svg` adds a wordmark variant
  for places that want the brand name alongside the icon.
- README hero replaces the PNG `<img>` with the SVG so contributors
  reading on GitHub light see a tinted version that doesn't blow
  out the page background.

- **8 production runtimes** named explicitly throughout: Claude
  Code, Hermes, Gemini CLI, LangGraph, DeepAgents, CrewAI, AutoGen,
  OpenClaw. Comparison table grew Hermes 4 + Gemini CLI rows with
  the integration mechanism (Option B upstream hook, A2A bridge,
  multi-provider derivation).
- **Canvas v4** — warm-paper theme system (light / dark / follow-
  system) called out alongside the existing Next.js 15 / React Flow /
  Zustand stack.
- **Memory v2 backed by pgvector** — semantic recall callout in
  both the "memory model" pitch line and the runtime stack section.
- **RFC #2967 typed-SSOT A2A response path** named in the platform
  ship list + architecture diagram.
- **SaaS surface section** added — multi-tenant EC2 + Neon +
  Cloudflare Tunnels, WorkOS + Stripe, KMS envelope, tenant_resources
  audit + 30-min reconciler. Cross-links to molecule-controlplane.
- **molecule-mcp-claude-channel plugin** added — entry point for
  Claude Code users to bridge A2A traffic into a local session via
  MCP. Documents the standard marketplace install flow + multi-
  tenant config.
- **Architecture diagram** redrawn with Canvas → Platform → Postgres
  + Provisioner (Docker | EC2+SSM) layout, plus a SaaS control plane
  block.
- **Quick Start** repo URL fixed (`molecule-monorepo` → `molecule-core`),
  Go version bumped to 1.25, Python ≥3.11 noted.

- Deploy buttons + Quick Start URL all bump from the old
  `molecule-monorepo` name to the current `molecule-core`. Pre-fix
  these clicked through to a 404.

The provisioner refactor (`registry.go` deletion + RegistryPrefix
env-driven changes) that lived alongside an earlier draft of this
README on the `docs/readme-refresh-2026-05-06` branch is OUT of
this PR — that work shipped separately via #6. This branch is
docs-only so the review surface is small and the merge is reversible.

- `git diff staging --stat`:
  ```
  README.md                              | 75 +++++++++++++++++++++++-----------
  docs/assets/branding/molecule-icon.svg | 28 +++++++++++++
  docs/assets/branding/molecule-logo.svg | 17 ++++++++
  3 files changed, 97 insertions(+), 23 deletions(-)
  ```
- SVGs validated in a browser at light + dark `prefers-color-scheme`.
- All linked docs (./docs/index.md, ./docs/quickstart.md, ./docs/
  architecture/architecture.md, ./docs/api-protocol/platform-api.md,
  ./docs/agent-runtime/workspace-runtime.md, ./LICENSE, etc.) verified
  to exist on staging.

- README.zh-CN.md mirror — non-trivial translation work; file as
  separate issue if mirror is wanted.
- molecule-ai/.github org-profile README — Gitea has no equivalent
  to GitHub's org-profile surface, and the GitHub org is suspended.
  Skipped.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

docs/assets/branding/molecule-icon.svg

@@ -0,0 +1,28 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64">
+  <style>
+    .bg { fill: #0a1120; }
+    .accent { fill: #7fe8d6; }
+    .accent-stroke { stroke: #7fe8d6; }
+    @media (prefers-color-scheme: light) {
+      .bg { fill: #f5f7fa; }
+      .accent { fill: #1a8a72; }
+      .accent-stroke { stroke: #1a8a72; }
+    }
+  </style>
+  <rect class="bg" width="64" height="64" rx="14"/>
+  <g class="accent-stroke" stroke-width="2.4" stroke-linecap="round" fill="none">
+    <line x1="32" y1="32" x2="12" y2="14"/>
+    <line x1="32" y1="32" x2="52" y2="18"/>
+    <line x1="32" y1="32" x2="10" y2="40"/>
+    <line x1="32" y1="32" x2="54" y2="44"/>
+    <line x1="32" y1="32" x2="32" y2="56"/>
+  </g>
+  <g class="accent">
+    <circle cx="32" cy="32" r="6.5"/>
+    <circle cx="12" cy="14" r="3.5"/>
+    <circle cx="52" cy="18" r="3.5"/>
+    <circle cx="10" cy="40" r="3.5"/>
+    <circle cx="54" cy="44" r="3.5"/>
+    <circle cx="32" cy="56" r="3.5"/>
+  </g>
+</svg>