feature: OpenCode support — detection, settings, parsed events, token and cost accounting

Feature Description

Second agent on the agent integration contract, and the first non-Claude one. OpenCode is
open source, so it is the integration anyone outside the team can reproduce.

Proposed Solution

• Detect an installed OpenCode and locate its settings
• Patch it to use localhost:47600 and the Cortex CA; ask before editing, support revert
• Traffic parsed into typed events, not merely proxied
• Token counts and cost correct for its wire format; if it is not Anthropic-shaped, either
  parse it properly or emit a clearly-labelled partial event
• Session and cost metrics aggregate alongside Claude Code sessions, with field names
  agreed with per-session token and cost aggregation
• A tool inventory for OpenCode, or state explicitly that tool pruning is unsupported for
  it — without one, pruning silently saves nothing
• Setup documented; verified depth stated

Additional Context

Part of the Cortex v0.9.0 release. Depends on the agent integration contract.