This PR turns openclaw-weixin into a native-only Rust channel for MicroClaw.

What is native now:

• QR login via ilink/bot/get_bot_qrcode + get_qrcode_status
• credential persistence per account
• long polling via ilink/bot/getupdates
• webhook-compatible inbound normalization
• text replies via ilink/bot/sendmessage
• image / video / file attachment replies via native getuploadurl + AES-128-ECB + CDN upload + sendmessage
• persisted context_token cache per user
• persisted get_updates_buf sync cursor
• CLI operations: microclaw weixin login|status|logout

What was removed:

• Weixin bridge mode
• Weixin auto mode
• Weixin send_command config path
• Node bridge examples and sidecar docs
• Weixin-specific MICROCLAW_WEIXIN_* send-command contract

Result:

• MicroClaw no longer requires Node or @tencent-weixin/openclaw-weixin for Weixin login, polling, text send, or attachment send.
• The Weixin channel is now a pure Rust implementation in MicroClaw.

Validation:

• cargo build
• cargo test weixin --lib
• cargo test --bin microclaw
• cargo test infer_channel_from_prefixed_chat_type -p microclaw-channels
• node scripts/generate_docs_artifacts.mjs --check