Add marketplace.json to enable /plugin marketplace install

[ydesgagn]

Summary

The README instructs users to install this plugin via /plugin marketplace add cloud-officer/claude-code-plugin-dev followed by /plugin install co-dev@cloud-officer, but the first command fails because the repo has no .claude-plugin/marketplace.json. Claude Code's marketplace flow requires a marketplace manifest in the repo it is asked to add. This PR adds a single-plugin marketplace manifest that names the marketplace cloud-officer and lists co-dev (with source: ".") as its sole plugin, making the documented install commands work end-to-end without any change to the README.

Key changes:

• Add .claude-plugin/marketplace.json declaring the cloud-officer marketplace with co-dev as an in-repo plugin (source: ".")

Types of changes

• Bugfix (fixes an issue)
• New feature (adds functionality)
• Refactoring (improves code without changing functionality)
• Breaking change (incompatible changes)
• Build or security update (updates dependencies, libraries, or security patches)
• Code style or documentation update (formatting, renaming, or documentation changes)
• Other (please describe):

Checklist

• Unit tests added to validate my fix/feature
• I have manually tested my change
• I did not add automation test. Why ?: Configuration manifest only; no executable code paths to cover.
• Database changes requiring migration with downtime or reprocessing of existing data
• The SOUP file lists the risk Level, requirements and verification reasoning associated with each library
• `readme.md` includes sections on introduction, installation, usage, and contributing
• `docs/architecture.md` includes sections on the architecture diagram, software units, software of unknown provenance, critical algorithms and risk controls related to PII and security
• Impact on PII, privacy regulations (CCPA/GDPR/PIPEDA), CIS benchmarks or security (availability/confidentiality/integrity); management must be notified