Fix OpenChamber /plan command by normalizing planner model

[fjmrytfjsn]

Summary

• Normalize planner agent model at startup via OPENCODE_PLANNER_MODEL (default: sonnet)
• Mitigate /plan failures caused by planner model/profile mismatch
• Document OPENCODE_PLANNER_MODEL in .env.template

Root cause

• /plan routes through planner agent, and planner-specific model settings can diverge from the active provider profile
• That mismatch caused /plan execution failures in OpenChamber

Files

• .devcontainer/startup.sh
• .env.template

Validation

• bash -n .devcontainer/startup.sh
• Runtime check: startup log includes "planner agent モデルを設定: sonnet"
• Runtime check: /config reflects planner model as sonnet after startup
• Planner agent smoke response confirmed