docs: publish feedback and private security contact channels

[hetaoBackend]

Change

Reporters in #359 and #300 had to open public issues to request a private vulnerability reporting channel. Publish the existing MiniMax Code contact channels in both READMEs: GitHub Issues, general support email, dedicated security email, Discord, the Feishu feedback group, and the X account for updates. Update SECURITY.md to direct private reports to security.mcode@minimax.io without requiring a public issue first.

Validation

• PASS: git diff --check; English/Chinese contact-link parity; public Discord redirect and Feishu QR image availability.
• PASS: pnpm verify (full profile, macOS arm64, Node.js 26.4.0) on commit d570a36 with a clean tracked worktree.
• Performance: basic; no runtime changes. Remote CI/performance results are separate from local verification.
• NOT RUN: Linux/Windows execution, email delivery, or joining community groups.

Publication and contribution checks

• I have permission to contribute these changes under the existing licenses applicable to the changed files/packages; imported material and its provenance are identified and existing notices are preserved.
• No credentials, account data, real user content, internal source history or private review material is included.
• No files were added or removed; source inventory and test declarations do not need regeneration.
• Shared English/Chinese documentation is updated. Offline checks are not described as live-service acceptance.

Maintainer handoff

Publication scope or license changes: none; existing user-facing contact channels are documented.

Shared-source port: not needed; documentation changes are specific to this public repository.

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