[Feature]: mark opencode as multi-install safe

[youngjoel]

Problem Statement

I've been switching back and forth between codex and opencode with spec-kit for a while now using

  specify integration install opencode --force
  specify integration use opencode

with no observed issues.

With Spec Kit 1.0.4:

specify integration install opencode

is correctly rejected when Codex is already installed. Installing with:

specify integration install opencode --force

creates a project where specify integration status --json reports:

{
  "status": "error",
  "installed_integrations": ["codex", "opencode"],
  "multi_install_safe": false,
  "findings": [
    {
      "code": "unsafe-multi-install",
      "message": "Installed integrations are not all declared multi-install safe: opencode"
    }
  ]
}

Reproduction And Observed Invariants

In a newly initialized, isolated fixture, we verified:

1. Codex uses .agents/skills/... and OpenCode uses .opencode/commands/....
   Their agent roots and command directories are distinct.
2. Forced OpenCode installation created the expected OpenCode command files
   without removing Codex skill files.
3. Spec Kit created separate manifests:
   .specify/integrations/codex.manifest.json and
   .specify/integrations/opencode.manifest.json.
4. specify integration use opencode and specify integration use codex both
   completed while retaining both integration roots.
5. Repeating integration use for either selected default did not change the
   fixture file manifest.
6. integration status --json reported no missing or modified managed files
   for either integration. The only reported error was OpenCode's missing
   multi-install-safe declaration.

This aligns with the published multi-install-safe criteria: static, distinct
agent roots and command directories; separate install manifests; stable command
invocation configuration; and no overlapping managed integration files.

Proposed Solution

Mark opencode as multi-install safe

Alternatives Considered

• Don't mark it as multi-install safe.
• Work with opencode upstream to make it multi-install safe

Component

Agent integrations (command files, workflows)

AI Agent (if applicable)

opencode

Use Cases

Working in projects where some devs prefer one agent vs another. Even a single dev who prefers say the codex experience however sometimes needs to use no-openai models well so switches temporarily to opencode

Acceptance Criteria

No response

Additional Context

No response

AI Disclosure

Some of the compatibility testing was done by gpt-5.6-terra at high in human-supervised mode.

[youngjoel]

Note that opencode testing was most recently done with version 1.18.31 from https://github.com/anomalyco/opencode