docs: fix SMTP username guidance

[asim48-ctrl]

Summary

• align the SMTP configuration table with the working Python and TypeScript examples
• clarify that the SMTP username should be the inbox email address, not agentmail
• add a troubleshooting row for authentication failures caused by the old username value

Fixes #148.

Verification

• git diff --check

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Summary by cubic

Clarifies in SMTP docs that the username is your inbox email address (e.g., myinbox@agentmail.to), not agentmail. Aligns the configuration table with the Python/TypeScript examples and adds a troubleshooting tip for auth failures; fixes #148.

^{Written for commit 43678fb. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 1 file

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