[mwbrooks]

Summary

This pull request gives developers who build this sample with a coding agent a short AGENTS.md with the guidance an agent can't work out from the code, and points every agent to the README's install steps for the Slack plugin.

• AGENTS.md is read by most coding agents. CLAUDE.md is one line, @AGENTS.md, so Claude Code reads the same file.
• It keeps only how we want agents to work: the checks to run before finishing and our code standards.
• A Slack Plugin section links to the README's "Claude Code, Codex, Cursor, and Other Coding Agents" section instead of repeating the install commands.
• Same shape as docs: add AGENTS.md for coding agents bolt-js-starter-agent#156.

Testing

• Each link's anchor matches the install section heading in the README next to it.
• The before-you-finish commands match the ones CI runs for this repo.
• The @AGENTS.md import is the same one checked with claude -p (Claude Code 2.1.304) on docs: add AGENTS.md for coding agents bolt-js-starter-agent#156.