Exact match for subject and recipient on search and sent

[heredia21]

Context

hey search subject matching is fuzzy. A query for a distinctive subject can return a different thread, and a short subject can return every message that shares those words. --to is the practical lookup after a send, and it is also not a strict equality check a script can trust when several threads share a recipient.

#469 asks for a sent listing with --subject, --to, and --from. Those flags need an exact mode. If they inherit fuzzy search, a script still cannot diff "this recipient + this subject" against outbound mail.

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Ask

• Add an exact mode for subject and recipient, on hey search and on the sent listing from Add a Sent / outbound listing (search and box cannot list mail you sent) #469. A flag such as --exact, or matching the full string, is enough.
• Exact subject matches the full subject, not a substring and not a related thread.
• Exact --to matches the envelope recipient, not a contact name or a nearby address.
• Document that the default remains fuzzy so current queries do not change.

Why it matters

After a batch, the reconciliation query is "this address, this subject, since this time." Fuzzy results make that query look successful when it hit the wrong thread.

Related: #469, #470.