feat: add OpenCode execution foundation

[pacphi]

Summary

First implementation slice for #76. It establishes the host-neutral execution boundary while preserving ak dual behavior and keeping OpenCode non-routable.

• add ADR-0018 and a normalized worker-result/execution-adapter contract
• materialize a host-neutral plan, then project it back into the existing Claude/Codex dual-run shape
• add an OpenCode loopback HTTP/SSE adapter with ephemeral Basic auth, isolated sessions, permission abort, and owned-server cleanup
• add an invocation-only OpenCode worker template; it does not alter user agents, permissions, or configuration

Safety

• no --auto or permission-policy mutation
• OpenCode remains canRouteActivities:false
• no provider, billing, or vendor-diversity fact is inferred from the host or model selector

Verification

• pnpm run check

Follow-up within #76: connect the generalized runner and prove live sandbox mutation, cancellation/timeout, malformed-event handling, and final-head evidence before the capability gate changes. #73 remains the next ordered issue after this PR merges.