[codex] Add Slack connector agent guidance

[koizumikento]

Summary

• Add a repo-scoped Codex agent skill for evaluating Slack Platform connector functions before implementing custom external API authentication.
• Add connector reference notes, an adoption checklist, and repo-specific implementation guidance under .agents/skills/slack-connector-functions/.
• Introduce AGENTS.md as the shared agent instruction file and reduce CLAUDE.md to an @AGENTS.md import stub, following Claude Code's documented import behavior.

Impact

This gives coding agents a clear decision path for proposing Slack connector functions when users request external SaaS integrations, while preserving Claude Code compatibility through CLAUDE.md.

Validation

• Reviewed generated skill and reference files.
• Verified AGENTS.md and CLAUDE.md layout locally.
• No Deno checks run because this PR changes documentation and agent instruction files only.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Relax mandatory MCP-tool requirement in shared AGENTS.md

This file now declares itself as the common rule set for Codex/Claude/Cursor, but it requires Serena/Context7 with no fallback (必ず使用). In any environment where those MCP servers are not installed or enabled, agents can dead-end before doing repository work, which is a functional regression from tool-agnostic instructions. Please make these tools preferred when available and document a fallback path (e.g., rg + official docs) so tasks remain executable.

Useful? React with 👍 / 👎.

[danielvo594520]

LGTM