fix(agent): initialize plugins before resolving config

[quangtran88]

Summary

• initialize plugins before Agent snapshots cfg.agent
• align agent resolution with provider resolution and project bootstrap expectations
• fix entry points that resolve plugin-provided default agents before full bootstrap

Closes#25438.

Why

Plugins can mutate config inside their config() hook. Agent.Service was reading config.get() before plugin initialization was guaranteed, so plugin-provided agents could be missing during early agent resolution.

Validation

• changed file has the minimal targeted diff in packages/opencode/src/agent/agent.ts
• package-level typecheck was attempted from packages/opencode
• current origin/dev still has unrelated baseline typecheck failures outside this change, including:
  • ../core/src/cross-spawn-spawner.ts: encoding type mismatch
  • src/storage/db.node.ts: missing node:sqlite types in the current environment

Scope

• packages/opencode/src/agent/agent.ts

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