v9.4.2: explicit outbound follow-up recipients

[the-code-learner]

Summary

• keep reply_email / create_reply_draft for inbound messages and reject clearly outbound messages with guidance to use follow-up
• add follow_up_email / create_follow_up_draft for outbound/Sent messages
• share recipient/thread resolution between reply and follow-up
• preserve original visible outbound To/Cc, remove sender-owned identities, deduplicate case-insensitively, and never rediscover source Bcc
• preserve normalized Re:, In-Reply-To, and extended References
• reuse the existing v9.4 tracked-delivery + clean-Sent pipeline and existing recipient authorization
• add v9.4.2 regression coverage and docs/version metadata

Safety / compatibility

• no direct changes to recipient allowlist/Sent-history policy
• no second tracking pipeline
• no new environment variables, volumes, ports, public callback paths, or Cloudflare requirements
• postmaster-mcp.yml is unchanged (same blob as main)
• public diff contains no real mailbox addresses/domains from the reported case

Tests

Local targeted resolver suite: 8/8 passing; new files compile. Required full repository regression suite is expected from v9 runtime tests / test on this PR.