fix: restore context-mode MCP

[xnoto]

Summary

• Restore the client-local context-mode MCP declaration in opencode.json with the same bare-binary launch contract used by claude-config and codex-config.
• The declaration was deleted in 570f62bf6c6365646ea228feff60b54e04f65207 as part of a broader MCP modernization; the commit gives no context-mode-specific rationale and no related PR record was found.
• Owner-directed exception: the request explicitly requires re-enabling the existing client-local MCP to match Claude/Codex, so it remains enabled: true despite the generic global disable-by-default guidance. mcp-gateway documentation explicitly classifies context-mode as client-local.

Type of change

• Bug fix
• Feature / enhancement
• Documentation
• Infrastructure (OpenTofu root or module)
• GitOps desired state (manifests, kustomize, charts, SOPS/KSOPS secrets)
• Container image
• CI / reusable workflow
• Refactor / cleanup
• Breaking change

Validation

• Required pull-request checks pass
• Generated or centrally distributed files were regenerated by their owning automation, not hand-edited

The PR workflow ran pre-commit run --all-files successfully, including JSON parsing and check-jsonschema against the upstream OpenCode schema. The schema permits this local-MCP shape. CI cannot prove that the context-mode executable exists on PATH, starts successfully, or completes an MCP handshake; those remain manual workstation verification after an owner-approved config update and OpenCode reload. No generated or centrally distributed file exists for this change: dotfiles consumes this canonical repository as a Git external.

Impact and rollout

Producer: xnoto/opencode-config. Consumer: OpenCode after the xnoto/dotfiles external Git checkout at ~/.config/opencode is manually updated and OpenCode reloads. No generated files, gateway, release, or GitOps changes.

Authored: this PR. Validated: CI passed. Published: not applicable. Installation and functional verification: manual and not performed. Rollback is removal of this five-line declaration.

Safety and secrets

• Contains no plaintext secrets, decrypted SOPS values, state files, kubeconfigs, tokens, or private endpoints
• No local OpenTofu init/plan/apply/destroy/import/state operations were run or claimed — plans come from pull-request checks
• Breaking or irreversible effects are described above with rollback notes

AI-assisted change; an adversarial review and QA coverage assessment were performed before opening this PR.