#80 contact-researchers: add the Thunderbird account signature to IMAP drafts

Summary

• mailbox.py draft appends the signature of the Thunderbird account matching OUTREACH_EMAIL. Thunderbird doesn't add the signature to a draft that is opened from the drafts folder.
• It reads the signature from prefs.js (the inline signature, or the signature file when one is attached). The profile is found in ~/.thunderbird, snap, flatpak or the macOS location. OUTREACH_THUNDERBIRD_PROFILE overrides the profile, and none turns the signature off.
• The signature goes after Thunderbird's -- separator, unless the account suppresses it. An HTML signature makes the draft multipart/alternative (plain text + HTML), so it opens in Thunderbird's HTML editor.
• mailbox.py config and draft print which signature was used.
• SKILL.md: when a signature is found, the body ends at Cheers,. The report also notes that Thunderbird blocks remote images in a signature (such as a logo) in stored drafts until remote content is allowed for the sender.

Test

• Built a test draft offline from the real Thunderbird profile and checked its plain-text and HTML parts.
• Replaced the 30 existing drafts in the outreach mailbox with signed copies, using with_signature, and checked that all 30 have the signature.

🤖 Generated with Claude Code