chore: convert vault.yml from age to ansible-vault encryption

- vault.yml was age-encrypted but scripts use ansible-vault commands
- Converted to ansible-vault format so vault.sh and get-secret.sh work
- Added encryption format change log to docs/secret-vault.md
- Verified: vault.sh view and get-secret.sh both working

secrets/vault.yml

@@ -1,5 +1,14 @@
-age-encryption.org/v1
--> X25519 a7BFvW8O4ym7K2fN/BMv7JiV+9iS1Tskb00yvfnzJEM
-yaap5PV+NYcVZ+gwlaWCbi1NFZ8OCJtha7ZGu/J5JGs
---- 4mxICGRpvAEL1gRxqTxRC60I6SRzYwBQ9cNye2cyDWA
-דôqÙÅy ?²^éP&&·Ÿ
§ŒÕLˆ�Š²~ÁßX7%fÂåå®›i­?’‡$8ÉPìèkål¦ùEÕƽø�<-"ï÷µL-†/O¼‡WÂéS+F»'||Œ§àÇÛ…¤XiA/5O+ÎÎvfÒ6‰áA+g³ÿ9‰|K˜`VÐJ´ù¶U‹Obd¬Ý¦äÄ)Âï›ýòìpãeÅÛ+áU¹2j²AÞ›8SÁô…ãujéñ¤æsÕŠ	Ϻ§Îu}ÝAâ…Y™ú»ø�謖
+$ANSIBLE_VAULT;1.1;AES256
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+3133363638383761303531303432316438333734366231390a303039653336633836303338333766
+32323437623635313363623730383562626464346333666437656466633832663663303236623138
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+32616663306336376131396633626139326663623435626161643364626239303432653135386439
+32656530353332326466663534373765333531383137623032343831363264636162663463626261
+33633062333463616137353564353535366135643666643730326264393039663966306635333962
+34323538663464633832633332653735353434626636393834346437613437646435333062353330
+62633463316432363131313532376338633661373133393136363232653636386465666464316537
+39313430636138376537643665643735393838306636363337356562363334383632656331666163
+64636337613531343364313234326137336239306631366531633232383262623364343936643164
+30373336653239613130613439303464393937626462633665303866323638636530633930336465
+62653662373533643533666662393632366263333239623933646662646665663465