Default sender when an account has several send-as addresses

[heredia21]

Context

#353 added hey compose --from, and that flag works: the CLI resolves a configured sender and sends with its id. What is still missing is a default.

hey account senders can list several addresses with none marked default, and the active account selection can be all. A compose that omits --from then depends on whichever identity the CLI picks. Scripts that pass --from on every call are safe. Anything that forgets the flag is not, and there is no way to set the sender once for an account.

This is not about connecting a new SMTP sender (#454) and not about search failing to find mail sent as an alias (#470).

No account-specific details below.

Ask

• hey account senders shows which sender is the default, if one is set.
• A command sets that default per account, for example hey account senders default <id-or-email>.
• hey compose and hey reply use that default when --from is omitted.
• When no default is set and more than one sender exists, compose fails with a structured error telling the caller to pass --from or set a default. Do not silently pick one.

Why it matters

Automation should fail closed when the From address is ambiguous, and should not have to repeat the sender id on every call once a default is chosen.

Related: #353 (closed), #454, #470.