fix: resolve mise environment for OpenCode plugin enablement

[disintegrator]

Summary

• Read GRAM_OPENCODE_PLUGINS through mise env --json using a shared isPluginEnabled helper in .opencode/lib/plugins.js, reused by the pause-stack and Worktrunk plugins and parsing JSON directly in JavaScript.
• Preserve exact colon-separated plugin ID matching and keep plugins disabled when the value is unset or lookup fails.
• Cover enablement and failure cases for both plugins, and update the documented append command to preserve resolved values.

Motivation

The previous mise set lookup read configured values without resolving the environment correctly. Using mise's resolved environment honors local overrides and templates without adding a jq subprocess to plugin setup.

[changeset-bot]

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Latest commit: 171d4d6

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[cubic-dev-ai]

No issues found across 5 files

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[gram-bot]

Auto-approved: review:bypass label applied by @disintegrator. Required status checks still gate this merge.