opencode: OpenCode 2 entry for the custody plugin, key substituted in place (parked reference, not for merge) - #76

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Waiting for `design-approved` on #75. This PR needs a linked issue with the `design-approved` label before it can be reviewed or merged. Link the approved issue with `Approved issue: #<issue>` (or `Refs #<issue>`) in the description. A maintainer will apply the `design-approved` label on the issue, or `trivial` on the pull request when there is genuinely no design to agree. The issue stays open until the change ships; maintainers take care of it then.

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