Thunderbird mbox grep: extract subjects before reading bodies

When searching a large local Thunderbird IMAP mbox for a topic, first grep with LC_ALL=C and -a for '^Subject:' lines across every account folder, then use the matching line numbers to read only those messages. Broad body greps return huge counts because terms match headers and encoded blobs. Short acronyms are especially noisy, and a case-insensitive pattern can match inside unrelated words such as 'address' for 'dres', so review the subject list before trusting a count. Thunderbird folder names also vary by account (for example 'Sent' rather than 'Sent Items'), so list the directory before assuming a path.