docs: fix stale channel-install instructions + dead Molecule-AI GitHub-org paths (#230)

The molecule-mcp-claude-channel install instructions in molecule-core
docs pointed at a non-existent `claude --channels` flag and the
github.com `Molecule-AI` org shorthand, which was suspended 2026-05-06.
Both forms no longer resolve and block users setting up the channel.

Replaced with the canonical Gitea marketplace flow (matches the channel
repo's own README):
  claude plugin marketplace add https://git.moleculesai.app/molecule-ai/molecule-mcp-claude-channel.git
  claude plugin install molecule@molecule-channel

- CONTRIBUTING.md:195  — wrong `--channels` one-liner + dead org shorthand
- README.md:241        — `/plugin marketplace add Molecule-AI/…` GitHub shorthand
- README.zh-CN.md:240  — same, Chinese localization

Docs-only. No code/workflow changes. Go import paths
(`github.com/Molecule-AI/molecule-monorepo/...`) intentionally left
unchanged: that is the `go.mod` module identifier used by all real code
in the repo, not a fetchable URL.

CONTRIBUTING.md

@@ -192,7 +192,7 @@ live in their own repos:
 
 - [`Molecule-AI/molecule-ai-workspace-runtime`](https://git.moleculesai.app/molecule-ai/molecule-ai-workspace-runtime) — Python adapter SDK (`molecule_runtime`) that runs inside containerized Molecule workspaces. Bridges Claude Code SDK / hermes / langgraph / etc. → A2A queue.
 - [`Molecule-AI/molecule-sdk-python`](https://git.moleculesai.app/molecule-ai/molecule-sdk-python) — `A2AServer` + `RemoteAgentClient` for external agents that register over the public `/registry/register` flow.
-- [`Molecule-AI/molecule-mcp-claude-channel`](https://git.moleculesai.app/molecule-ai/molecule-mcp-claude-channel) — Claude Code channel plugin. Bridges A2A traffic into a running Claude Code session via MCP `notifications/claude/channel`. Polling-based (no tunnel required); install with `claude --channels plugin:molecule@Molecule-AI/molecule-mcp-claude-channel`.
+- [`molecule-ai/molecule-mcp-claude-channel`](https://git.moleculesai.app/molecule-ai/molecule-mcp-claude-channel) — Claude Code channel plugin. Bridges A2A traffic into a running Claude Code session via MCP `notifications/claude/channel`. Polling-based (no tunnel required); install via the marketplace flow: `claude plugin marketplace add https://git.moleculesai.app/molecule-ai/molecule-mcp-claude-channel.git` then `claude plugin install molecule@molecule-channel`. (The old `claude --channels plugin:molecule@Molecule-AI/…` one-liner is dead — `--channels` is not a real flag and the GitHub `Molecule-AI` org was suspended 2026-05-06; see the channel repo README for the full setup guide.)
 
 When extending the **A2A surface** in molecule-core (`workspace-server/internal/handlers/a2a_proxy.go` etc.), consider whether the change has a downstream impact on the runtime SDK or the channel plugin — they're versioned independently but share the wire shape.
 

README.md

@@ -238,7 +238,7 @@ The result is not just “an agent that learns.” It is **an organization that
 - subscribe to one or more workspaces; peer messages surface as conversation turns; replies route back through Molecule's A2A
 - no tunnel, no public endpoint — the plugin self-registers each watched workspace as `delivery_mode=poll` and long-polls `/activity?since_id=…`
 - multi-tenant friendly: one plugin install can watch workspaces across multiple Molecule tenants (`MOLECULE_PLATFORM_URLS` per-workspace)
-- install via the standard marketplace flow: `/plugin marketplace add Molecule-AI/molecule-mcp-claude-channel` → `/plugin install molecule-channel@molecule-mcp-claude-channel`
+- install via the standard marketplace flow: `claude plugin marketplace add https://git.moleculesai.app/molecule-ai/molecule-mcp-claude-channel.git` → `claude plugin install molecule@molecule-channel` (the old `Molecule-AI/…` GitHub shorthand no longer resolves — that org was suspended 2026-05-06; use the full Gitea URL)
 
 ## Built For Teams That Need More Than A Demo
 

README.zh-CN.md

@@ -237,7 +237,7 @@ Molecule AI 并不是要替代下面这些 framework，而是把它们纳入更
 - 订阅一个或多个 workspace；peer 的消息会以 user-turn 出现，回复会经 Molecule A2A 路由出去
 - 无需公网隧道、无需公开端点 —— 插件启动时自动把每个 watched workspace 注册成 `delivery_mode=poll`，长轮询 `/activity?since_id=…`
 - 多租户友好：单次安装即可同时 watch 跨多个 Molecule 租户的 workspace（`MOLECULE_PLATFORM_URLS` 按 workspace 配置）
-- 通过标准 marketplace 流程安装：`/plugin marketplace add Molecule-AI/molecule-mcp-claude-channel` → `/plugin install molecule-channel@molecule-mcp-claude-channel`
+- 通过标准 marketplace 流程安装：`claude plugin marketplace add https://git.moleculesai.app/molecule-ai/molecule-mcp-claude-channel.git` → `claude plugin install molecule@molecule-channel`（旧的 `Molecule-AI/…` GitHub 简写已失效——该组织于 2026-05-06 被封停，请使用完整的 Gitea URL）
 
 ## 适合什么团队