Record the session-per-notebook change in the changelog

[jgfoster]

Follows #5, which changed behaviour a user can see and left the entry for the release step. Writing it now rather than at release, since the release only catches what it is told about.

Three parts, because the change has three faces:

• Changed — two notebooks no longer share variables or a transaction, and the reason is stated as the bug it fixes: a commit() in one used to commit another's half-finished work, and with gemdb.transaction(): refused to start whenever any open notebook had left the shared session dirty.
• Fixed — reinstalling Python support, uninstalling, or changing the root path now log out every session, so no notebook is left holding a view of a database that has been replaced.
• Known limitations — notebooks now spend a scarce resource. Ten sessions, minus the database's own gems, minus every open GemDB Shell, is roughly six or seven notebooks. That belongs in the changelog because it is the one way this change can bite, and the refused-login message is the answer to it.

Nothing outside CHANGELOG.md is touched. #4 is not listed: it added docs/reaching-windows.md, which .vscodeignore keeps out of the .vsix, so it changes nothing a user of the extension can observe.

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