feat: add project-idea-validator subagent

[ScientificAJ]

Summary

• add a new project-idea-validator subagent under 10-research-analysis
• update the main catalog and category README with the new agent entry and selection guidance
• bump the voltagent-research plugin version and sync the marketplace entry

Why

This adds a dedicated specialist for adversarial go/no-go product validation before roadmap work begins. It combines competitor teardown, demand checking, uniqueness pressure-testing, and forced pivot logic into a single idea-gate workflow.

Overlap and distinction

This sits near existing agents like product-manager, market-researcher, and competitive-analyst, but it is intentionally narrower and not redundant.

• product-manager is broader product strategy and roadmap work
• market-researcher focuses on market understanding
• competitive-analyst focuses on rival intelligence
• project-idea-validator exists to pressure-test whether an idea should be built at all