opencode: enable session resume by default and fix prompt re-delivery

[mafredri]

Problem

Two issues:

1. Resume is off by default. The module has full resume wiring (--continue, --session, --type opencode all present in start.sh) but the continue variable in main.tf defaults to false. Out of the box, sessions are never resumed.

2. Prompt is re-sent on resume. In build_opencode_args(), the initial prompt is added to AGENTAPI_ARGS via -I "$PROMPT" unconditionally when ARG_AI_PROMPT is set, regardless of whether ARG_CONTINUE is true. When a session is resumed, the task prompt gets re-injected into an already-active conversation, potentially re-triggering the original task.

To reproduce: set continue = true and ai_prompt = "some task". Start the module, have a conversation, stop, restart. The agent resumes the session but the task prompt is re-sent as a new message.

OpenCode's --continue gracefully creates a new session when no prior sessions exist, so the default flip is safe for first-run scenarios.

Desired outcome

• continue defaults to true.
• The initial prompt is only sent on cold start, not when resuming an existing session.
• After feat(coder/modules/agentapi): add state persistence #736 lands: enable_state_persistence = true so the UI matches the agent's memory.

Version bump

minor