[FEATURE] Add Claude Code as a supported agent surface (.claude-plugin manifests)

[vmlepaulino]

Feature Description

Make the Spec2Cloud lean and speckit plugins installable in Claude Code (Anthropic's agent CLI), alongside the existing GitHub Copilot support — without changing or duplicating the skill content.

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Problem Statement

Today the plugins are surfaced only through the GitHub Copilot plugin marketplace (copilot plugin marketplace add Azure-Samples/Spec2Cloud) and APM. The skill content itself (the SKILL.md files with name/description frontmatter under each plugin's skills/ directory) is already in the portable format that Claude Code reads natively. The only thing preventing Claude Code users from installing the toolkit is the manifest location: Claude Code discovers plugins via .claude-plugin/marketplace.json (repo root) and .claude-plugin/plugin.json (per plugin), whereas this repo places them under .github/plugin/ (the Copilot convention).

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Proposed Solution

Additive only — leave the Copilot path (.github/plugin/...) completely untouched:

1. Add /.claude-plugin/marketplace.json at the repo root, porting the existing .github/plugin/marketplace.json so users can run /plugin marketplace add Azure-Samples/Spec2Cloud in Claude Code.
2. Add .claude-plugin/plugin.json to each plugin (plugins/lean-spec2cloud/, plugins/speckit-spec2cloud/), mirroring the existing .github/plugin/plugin.json.
3. Add a "Use with Claude Code" section to the README documenting the install commands.

The existing skills/ directories are discovered automatically by Claude Code, so no skill files need to change.

Possible follow-ups (out of scope for the first PR)

• A package-*-claude-plugin skill mirroring the existing package-speckit-spec2cloud-plugin skill, so the Claude manifests stay generated/in-sync with source.
• Neutralizing the few Copilot-only content references (/fleet usage in the README, also emitting CLAUDE.md next to .github/copilot-instructions.md).

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Alternative Solutions

• Local-only binding (drop skills into ~/.claude/skills/): works for individuals but isn't shareable/installable, so it doesn't serve the catalog's distribution goal.
• Separate repo for Claude manifests: fragments the source of truth; keeping one repo with per-target manifests matches the existing pattern (Copilot + VS Code + APM already coexist here).

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Use Cases

• A developer who uses Claude Code wants the same Azure spec → plan → implement → verify → deploy loop without switching to Copilot.
• A team standardizing on Claude Code can /plugin marketplace add Azure-Samples/Spec2Cloud and get the lean/speckit skills, identical to the Copilot experience.

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Benefits

• Broader reach: one toolkit, multiple agent harnesses (Copilot, VS Code, APM, and now Claude Code).
• Zero duplication: same SKILL.md source serves all targets.
• Low risk: purely additive; no change to existing Copilot/VS Code behavior.

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Additional Context

Happy to implement this. Opening per CONTRIBUTING.md ("submit an issue with a proposal for your work first") — a PR with items 1–3 will follow shortly, scoped small and additive.