chore: align OpenCode plugin to 1.18.4

[aadil96]

What

• Align @opencode-ai/plugin with the OpenCode CLI at 1.18.4.
• Refresh the Bun lockfile for the matching plugin and SDK dependency graph.

Why

Keeping the plugin and CLI on the same release avoids API/schema skew while resolving the delegation failures caused by the older CLI reading the newer database schema.

Validation

• bun install --frozen-lockfile --ignore-scripts
• Installed plugin version resolves to 1.18.4
• git diff --check
• Fresh direct prompt, native task delegation, and custom delegate probes passed under CLI/plugin 1.18.4.