refactor(opencode): update plugin templates for v1 hooks

[Adamcf123]

Summary

Updates the OpenCode plugin templates under packages/cli/src/templates/opencode/ to match the current v1 plugin API and remove outdated OMO assumptions.

Adds a follow-up fix so template session-start context is deduped using OpenCode-managed session history instead of process-local or temp-file state, which prevents reinjection after reopening the same session.

Changes

• Convert template plugins to v1 format (export default { id, server })
• Add required id fields to the template plugins
• Remove dead OMO compatibility code from the template trellis-context.js
• Fix the task tool prompt mutation pattern in the template inject-subagent-context.js by using in-place args.prompt = newPrompt
• Fix the template session-start.js injection flow to use chat.message output mutation instead of the non-persistent experimental.chat.messages.transform path
• Simplify the template ContextCollector by removing unused cross-hook methods
• Add restart-safe template session-start dedupe by checking persisted session history through the OpenCode SDK
• Mark injected Trellis context with structured metadata and keep a legacy text-marker fallback for older sessions
• Add regression tests for the template history-based dedupe behavior

Scope

• packages/cli/src/templates/opencode/lib/trellis-context.js
• packages/cli/src/templates/opencode/plugins/inject-subagent-context.js
• packages/cli/src/templates/opencode/plugins/session-start.js
• packages/cli/test/templates/opencode.test.ts

Verification

• pnpm lint passes
• pnpm typecheck passes
• pnpm test passes