Add standalone dynamic invoice Sieve rule

The prompts used to arrive at this change since the previous commit

User reported the Dynamisch kwartaal Roundcube filter is empty and asked for a new file containing a dynamic invoice-quarter filter while keeping the rest of the plan approved.

Any special observations that may be relevant for version management for this version. Be brief.

invoice_dynamic_quarter.sieve uses Date header year capture plus regex month tests and compiles with sievec. Existing untracked financ.sieve, codex.session, roundcube.txt, plan.md, and verplaats_log.json were left untouched.

restart_prompt.md

@@ -70,6 +70,7 @@ Logs/state kept:
 Roundcube/Sieve:
 
 - `mailcat_roundcube.sieve` is the generated Roundcube/Managesieve Sieve file for the current routing setup.
+- `invoice_dynamic_quarter.sieve` is a standalone Roundcube/Managesieve Sieve file containing only the dynamic invoice-quarter rule. It uses the message `Date` header to set `jaar`, regex month tests to set `kwartaal`, then files into `INBOX.Financieel.Facturen.${jaar}.${kwartaal}`.
 - It uses `INBOX.`-prefixed mailbox names because the user's Roundcube export used `fileinto :copy "INBOX.Notes";`.
 - It includes `Afmelden.*` routes and uses `fileinto` without `:copy`, so matching incoming messages are sorted instead of duplicated.
 - Rules have Roundcube-compatible `# rule:[...]` names.