fix(adapters): detect OpenCode from MCP client info

[rahul05ranjan]

What / Why / How

Addresses the MCP clientInfo gap in #1255. OpenCode identifies itself as opencode, but that name was absent from the client map, so detection could fall through to another installed agent's directory. Mapping that name to the existing OpenCode adapter makes the explicit handshake win over ambient home-directory probes.

This does not change the fallback when a client provides no recognizable clientInfo; that broader detection policy remains open in #1255.

Affected platforms

• OpenCode

Test plan

• Added mapping and platform-detection tests, including another agent's environment marker.
• pnpm exec vitest run tests/adapters/client-map.test.ts tests/adapters/detect.test.ts — 101 passed.
• pnpm exec tsc --noEmit — passed.
• git diff --check origin/next...HEAD — passed.

Checklist

• Tests added/updated
• npm test passes (full suite not run; focused tests above passed)
• Typecheck passes via pnpm exec tsc --noEmit
• Docs reviewed; no user-facing instructions change
• No file path behavior changed
• Targets next