mail-mcp: resolve binaries via PATH + account filter on mail_inbox
Make the mail MCP work on macOS without hardcoded /usr/bin paths:
- Resolve mu, mbsync, msmtp via \`which\` with /opt/homebrew fallbacks
- Use \$HOME for maildir + xapian db
- Honor AC_MAIL_* env overrides so hosts with unusual layouts still work
Also add an account='ac-mail'|'jas-mail'|'all' filter to mail_inbox so both
accounts share one tool entry rather than requiring two separate calls.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>