docs: update version references and counts across markdown files

- AGENTS.md: version 3.0.0 → 3.1.0-dev
- CLAUDE.md: version v3.0.0 → v3.1.0-dev (2 places)
- ROADMAP.md: status v2.7.0 → v3.1.0-dev, 18 MCP tools → 26, 1200+ tests → 1,880+, add recent improvements
- docs/README.md: "New in v2.7.0" → "New in v3.x", 1200+ tests → 1,880+, docs version 2.7.0 → 3.1.0-dev, date updated

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

AGENTS.md

@@ -12,7 +12,7 @@ This file provides essential guidance for AI coding agents working with the Skil
 
 | Attribute | Value |
 |-----------|-------|
-| **Current Version** | 3.0.0 |
+| **Current Version** | 3.1.0-dev |
 | **Python Version** | 3.10+ (tested on 3.10, 3.11, 3.12, 3.13) |
 | **License** | MIT |
 | **Package Name** | `skill-seekers` (PyPI) |