[Feature]: Support multiple installed integrations in the same project

[PascalThuet]

Problem Statement

specify integration now supports install, uninstall, and switch, but it still appears to enforce a single installed integration per project.

I'm frustrated when a repository needs to support multiple agents used by different people on the same team, because the second integration cannot be installed alongside the first one.

Repro with specify 0.6.1:

specify init --here --ai claude --offline --force
specify integration install codex

Observed behavior:

Integration 'claude' is already installed.
Run specify integration uninstall first, or use specify integration switch codex.

Also:

• specify integration list shows only one installed integration
• .specify/integration.json still models a single current integration

This means #1924 / #2065 solved integration management and switching, but not simultaneous multi-install in one project.

Proposed Solution

Allow multiple integrations to be installed in the same repository, while keeping one integration as the default/active one.

Concretely, I would like:

• multiple installed integrations in one project
• one default_integration
• explicit installed integration state, for example installed_integrations
• specify integration install <agent> to work without requiring uninstall/switch first when another integration is already present
• specify integration use <agent> (or equivalent) to change the default integration without uninstalling others

Conceptually, something like:

{
  "default_integration": "claude",
  "installed_integrations": ["claude", "codex"]
}

Alternatives Considered

• specify integration switch <agent> works for one person changing tools, but it does not solve shared multi-agent repos
• uninstalling and reinstalling integrations works as a workaround, but it is awkward and not a good fit for teams using different agents in the same project
• re-running specify init with another agent can leave artifacts on disk, but the supported CLI flow still behaves as single-install

Component

Specify CLI (initialization, commands)

AI Agent (if applicable)

Claude Code

Use Cases

1. A team shares one repository, but some developers use Claude and others use Codex.
2. A project wants to keep both agent-specific command/skill setups available without forcing users to switch the whole repo state.
3. A repo is maintained over time by different contributors who do not all use the same AI agent, but still want to share the same .specify/ workflow.

Acceptance Criteria

• It is possible to install more than one integration in the same project
• One integration can be marked as the default/current integration
• Installing a second integration does not require uninstalling the first one
• The project metadata explicitly represents multiple installed integrations
• specify integration list shows all installed integrations clearly
• Documentation explains the difference between installed integrations and the default integration

Additional Context

Related issues:

• Multi agent in one project. #377
• Running multiple agents with spec-kit in one project? #1065
• From Agents to Integrations #1924
• Add specify integration subcommand for post-init integration management #2065

This request is not about replacing switch. switch is useful. The gap is support for repositories where multiple integrations need to coexist at the same time.