Add a new skill under plugins/advanced-security/skills/dependency-scanning/
that guides agents through scanning project dependencies for known CVEs and
security vulnerabilities using ecosystem-native audit tools.

Supported ecosystems:
- JavaScript: npm audit, yarn audit (v1 and Berry v2+), pnpm audit
- Python: pip-audit
- Rust: cargo audit
- Ruby: bundler-audit
- Go: govulncheck
- .NET: dotnet list package --vulnerable

The skill covers auto-detection of ecosystems via lock files, running the
appropriate audit command with JSON output, presenting findings grouped by
severity (critical -> high -> moderate -> low), and providing per-ecosystem
remediation commands.

Also fixes marketplace.json: spark plugin was referencing ./skills/spark
which does not exist; the correct path is ./skills/spark-app-template.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Introduce a new skill that scans project dependencies for known CVEs
and security vulnerabilities using the native audit tool for each
detected package manager.

Supported ecosystems:
- JavaScript: npm audit, yarn audit (v1 and Berry v2+), pnpm audit
- Python: pip-audit
- Rust: cargo audit
- Ruby: bundler-audit
- Go: govulncheck
- .NET: dotnet list package --vulnerable

The skill auto-detects ecosystems from lock files, runs the audit with
JSON output, reports findings by severity (critical, high, moderate,
low) with CVE IDs and fix commands, and optionally cross-references
open Dependabot alerts via gh api or the REST API.

Also registers the skill in marketplace.json and updates the
advanced-security plugin README.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>