feat(cli-init): propose additional agent init flow

openspec/changes/add-multi-agent-init/proposal.md

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+# Allow Additional AI Tool Initialization After Setup
+
+## Summary
+- Let `openspec init` configure new AI coding tools for projects that already contain an OpenSpec structure.
+- Keep the initialization flow safe by skipping structure creation and only generating files for tools the user explicitly selects.
+- Provide clear feedback so users know which tool files were added versus already present.
+
+## Motivation
+Today `openspec init` exits with an error once an `openspec/` directory exists. That protects the directory layout, but it blocks
+teams that start with one assistant (for example, Claude Code) and later want to add another such as Cursor. They have to create
+those files by hand or rerun `init` in a clean clone, which undermines the "easy onboarding" promise. Letting the command extend
+an existing installation keeps the workflow consistent and avoids manual file management.
+
+## Proposal
+1. Detect an existing OpenSpec structure at the start of `openspec init` and branch into an "extend" mode instead of exiting.
+   - Announce that the base structure already exists and that the command will only manage AI tool configuration files.
+   - Keep the existing guard for directories or files we must not overwrite.
+2. Present the usual AI tool selection prompt even in extend mode, showing which tools are already configured.
+   - Skip disabled options that remain "coming soon".
+   - Mark already configured tools as such so users know whether selecting them will refresh or add files.
+3. When the user selects additional tools, generate the same initialization files that a fresh run would create (e.g., Cursor
+   workspace files) while leaving untouched tools intact apart from marker-managed sections.
+   - Do nothing when the user selects no new tools and keep the previous error messaging to avoid silently succeeding.
+4. Summarize the outcome (created, refreshed, skipped) before exiting with code 0 when work was performed.
+   - Include friendly guidance that future updates to shared content still come from `openspec update`.
+
+## Out of Scope
+- Changing how `openspec update` discovers or updates AI tool files.
+- Supporting brand-new AI tools beyond those already wired into the CLI.
+- Adding non-interactive flags for selecting multiple tools in one run (follow-up if needed).
+
+## Risks & Mitigations
+- **User confusion about extend mode** → Explicitly log what will happen before prompting and summarise results afterward.
+- **Accidental overwrites** → Continue using marker-based updates and skip files unless the user chooses that tool.
+- **Inconsistent state if init fails mid-run** → Reuse existing rollback/transaction logic so partial writes clean up.

openspec/changes/add-multi-agent-init/specs/cli-init/spec.md

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+## MODIFIED Requirements
+### Requirement: Safety Checks
+The command SHALL perform safety checks to prevent overwriting existing structures and ensure proper permissions.
+
+#### Scenario: Detecting existing initialization
+- **WHEN** the `openspec/` directory already exists
+- **THEN** inform the user that OpenSpec is already initialized and skip recreating the base structure
+- **AND** continue to the AI tool selection step so additional tools can be configured
+- **AND** display the existing-initialization error message only when the user declines to add any AI tools
+
+## ADDED Requirements
+### Requirement: Additional AI Tool Initialization
+`openspec init` SHALL allow users to add configuration files for new AI coding assistants after the initial setup.
+
+#### Scenario: Configuring an extra tool after initial setup
+- **GIVEN** an `openspec/` directory already exists and at least one AI tool file is present
+- **WHEN** the user runs `openspec init` and selects a different supported AI tool
+- **THEN** generate that tool's configuration files with OpenSpec markers the same way as during first-time initialization
+- **AND** leave existing tool configuration files unchanged except for managed sections that need refreshing
+- **AND** exit with code 0 and display a success summary highlighting the newly added tool files

openspec/changes/add-multi-agent-init/tasks.md

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+# Implementation Tasks
+
+## 1. Extend Init Guard
+- [ ] 1.1 Detect existing OpenSpec structures at the start of `openspec init` and enter an extend mode instead of failing.
+- [ ] 1.2 Log that core scaffolding will be skipped while still protecting against missing write permissions.
+
+## 2. Update AI Tool Selection
+- [ ] 2.1 Present AI tool choices even in extend mode, indicating which tools are already configured.
+- [ ] 2.2 Ensure disabled "coming soon" tools remain non-selectable.
+
+## 3. Generate Additional Tool Files
+- [ ] 3.1 Create configuration files for newly selected tools while leaving untouched tools unaffected apart from marker-managed sections.
+- [ ] 3.2 Summarize created, refreshed, and skipped tools before exiting with the appropriate code.
+
+## 4. Verification
+- [ ] 4.1 Add tests covering rerunning `openspec init` to add another tool and the scenario where the user declines to add anything.