Fix omp zai.key encryption: use proper chezmoi 'encrypted_' attribute

The previous approach (private_dot_omp/agent/zai.key.age + manual
re-encryption) didn't work because:
1. The 'private_' prefix is for files NOT to push to remote, not for
   encrypted files. The 'encrypted_' prefix is what chezmoi recognizes
   as an encryption marker.
2. The encrypted file needs to be at dot_<path>/encrypted_<name>.age
   so chezmoi can both decrypt on apply AND strip the .age suffix
   to write the destination file as <name> (without .age).

Also fix chezmoi age config to actually decrypt non-interactively:
- Add useBuiltinAge: false to force external age binary
- Add age.command: /usr/bin/age (absolute path) so PATH issues
  don't matter in non-interactive SSH contexts

The encrypted file is at dot_omp/agent/encrypted_zai.key.age, decrypts
to ~/.omp/agent/zai.key on apply. Encrypted to all 6 recipients
(recovery + miche + byte + kaiser + rye + crouton).

Tested on miche:
  - chezmoi apply: rc=0
  - live zai.key: 50 bytes (correct content)
  - decrypts with miche per-machine key
  - would decrypt on other boxes with their respective keys

dot_omp/agent/encrypted_zai.key.age

@@ -0,0 +1,18 @@
+-----BEGIN AGE ENCRYPTED FILE-----
+YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSBja210UjFyaUdtSkw2M25G
+WWJ2N3ptejFCRGdMSFVnVk1IUjdxVkIzUldrClR6MjJraThHOFUzTklxZDFIM09B
+QzdsVVpHNFpaaVpJeUJNNVNtOUlGUlkKLT4gWDI1NTE5IDVWN1JjeGV1ZjFJUXNo
+bFRmdGd4WTFRZDBhaDBXQVliWVVacUUyemIvaXMKRksrOWwzUk8rY2hJcUN0enpW
+NXUvdHEvREVHRjFwQ0lmZkdzY2pDVE1lcwotPiBYMjU1MTkgQm9OYzVSWmpKc08x
+NGdjVWZFcS9GV253Q0k4RzVBK3JzSmRHU1gyWC95WQpsL3M0aEozcGI2RFpmNE50
+SFE0bTJnRnpQOXI0M2pSR2pGWEV4QnJMTTdNCi0+IFgyNTUxOSBBZ1I0TGhqY044
+bndZaVdSaW51c2ljc1l4ckV4N3haNStvaHRsNGpsV2lJCjBJNFhySUwxM2toSXVD
+ZkZEYkZzMXd6Mk0ycEdwRVZDcWg3djY5ZWF1QVEKLT4gWDI1NTE5IDdmQjB3RU05
+cFM0VTBzWUl0dVZwWVlKUmhDbmlwY0ZKbFdpTWIyRnZLbTgKVWFRejdvL1VEeHp0
+SnZ1YWJKQ3lSR1owTjdTUUt5Q0lKQjFEZkI2WW8xVQotPiBYMjU1MTkgUW5ZMWZC
+MTJBNGovTERvZXBXS2dSVVlYSW5heWQ3MTZBbWVYNmpmSWpFRQpoV2xsL3BUL1JK
+cXQvL0s0SGZ1ZzZESUZoZXJ5VzNVazJKWkxYMkJoZWhNCi0tLSBSa3VaNUNIOTgw
+VUFUZE41MWk2NEZrVE5xREc2NUVrd3hsOU15K0xxNm9rCuSszdd/l0WSCvWGiyJe
+w4fB7uhKLWBX1t+OByoNnh/ZImiRkGXOk/BiX3VaCn9tB5KigWzR+n6RhpepNVMf
+4Ouir24gMu+UIYo8NoYjHvxKuzg=
+-----END AGE ENCRYPTED FILE-----