mail_send: HEADER_SCAN_WINDOW=100 too small for reply-to-search on high-volume inboxes

[4444J99]

Hit in production 2026-07-16: --reply-to-search subject fragments could not find a 10-day-old thread because the client-side subject scan only covers the newest 100 All Mail messages (GitHub-notification volume pushes real threads out of the window in hours). Workaround used: resolve the RFC Message-ID via server-side SEARCH and pass (no window limit). Fix: make the window env-tunable (e.g. MAIL_SEND_SCAN_WINDOW) and/or try a server-side ASCII SUBJECT SEARCH first, falling back to the client-side unicode scan.

🤖 Generated with Claude Code

[4444J99]

Triage-only pass 2026-07-18 because UMA has active work in flight today: this is still current. Next concrete step: add an env-tunable scan window plus a server-side SUBJECT/Message-ID search fallback, with a focused regression proving high-volume inbox lookup no longer falls off the first 100 messages.