feat: add OpenCode adapter

[Ayaan-20-11]

Summary

• Add an OpenCode SQLite adapter for AgentBurn
• Read session-level token and cost accounting from opencode.db
• Register OpenCode with the AgentBurn adapter registry
• Track message counts and provider/model metadata
• Open the database read-only and support WAL-backed databases
• Detect zero-token sessions through existing AgentBurn accounting diagnostics

Validation

• Tested against a real OpenCode SQLite database
• agentburn --agent opencode report works
• agentburn --agent opencode --json report works
• agentburn --agent opencode doctor works
• Existing offline self-test: 184 tests passed
• Working tree clean after commit

[Ayaan-20-11]

Is it good ?