docs(config): declare Gmail and PonyMail mail sources

[potiuk]

Summary

• Declare the draft-only Gmail MCP (gmail-plaintext) as the primary mail source and the read-only PonyMail archive as fallback in the self-adoption .apache-magpie-overrides/project.md, both non-mandatory.
• Replaces the "none declared" row, so skills and the Copilot CLI isolated-setup install see the mail backends this project actually uses; roles match the ASF organization default (mail_provider).

Type of change

• Skill change (.claude/skills/<name>/) — eval fixtures updated below
• Tool / bridge contract (tools/<system>/*.md)
• Python package (tools/*/ with pyproject.toml)
• Groovy reference impl
• Cross-cutting (RFC, AGENTS.md, sandbox, privacy-LLM)
• Documentation (docs/, README.md, CONTRIBUTING.md)
• Project template (projects/_template/)
• CI / dev loop (prek, workflows, validators)
• Other: self-adoption project config (.apache-magpie-overrides/project.md)

Test plan

• Commit-time prek hooks ran on the changed file; the file-scoped hooks that apply (whitespace, line endings, SPDX) passed and the rest had no files to check.
• prek run --all-files — not run for this two-line table edit; CI runs the hooks scoped to the PR diff.
• Other: confirmed gmail-plaintext and ponymail are present in ~/.copilot/mcp-config.json.

RFC-AI-0004 compliance

• Write-access discipline — Gmail is declared draft-only; no autonomous outbound messages.

Linked issues

None.

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Was generative AI tooling used to co-author this PR?

• Yes — Copilot SDK in VS Code

Generated-by: Copilot SDK in VS Code