feat: bash completion for the launch selectors

Completes the options, and the account keys after --account, read from the
[account.<key>] sections of the config the application itself reads. Thread
and message ids are deliberately not completed: offering them means querying
the whole notmuch index on every Tab, which on a real Maildir is tens of
thousands of ids nobody would pick from a list anyway.

Installed under share/bash-completion/completions, where bash-completion
loads a command's file on demand by name.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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