Configuration migration for v0.6.0

Update ~/.config/notmutt/notification.toml to enable individual urgent alerts:

[notify]
tags = ["inbox", "unread"]

[notify.normal]
max = 3

[notify.important]
match-tags = ["important"]

The shared notify.tags scope requires every listed tag. Only messages changed by classification are notification candidates. Any eligible message carrying a notify.important.match-tags tag gets one urgent alert; the other candidates get one ordinary batch. Important alerts ignore notify.normal.max. Individual platform alerts show the sender as title and subject as popup body, without repeating the sender or exposing message-body text; they play a sound. Command backends can use {urgency} (normal or critical). Add importance through a [[filter.header-rules]] rule; notification config does not classify mail.

Existing [notify] priority remains accepted but only sorts batch headlines. Move those tags to [notify.important] match-tags if you want individual alerts. Existing [notify] max remains the ordinary cap unless [notify.normal] max is set. The new notification.toml template is created only when absent; startup never overwrites your edits.

Indexer migration: If a notmuch post-new hook or afew filter applies account or folder tags before notmutt runs, remove those rules. Notmutt’s filter engine owns that classification. Mail already carrying all applicable tags produces no operation and cannot trigger a notification. Other clients that depend on the old hook must arrange their own classification.

If your config still has [pager] image-protocol = "kitty", remove that obsolete key before upgrading. The client now detects terminal image support automatically, and strict config loading rejects the old key.