Refresh README and resources for 2026; add Latest Update Watchlist and validation updates

[requie]

Motivation

• Bring the guide and reference index up-to-date with recent 2025–2026 regulatory and standards developments and surface key update triggers.
• Make it easier for maintainers to track authoritative sources and effective dates for major frameworks and regulations.
• Keep the public-facing documentation badges and evidence tags consistent with the latest validations.

Description

• Updated README.md to bump the Updated badge to 2026, refresh wording to reflect 2024–2026 coverage, and add a new "Latest Update Watchlist (Validated: 2026-04-27)" section with EU AI Act, OWASP Agentic Top 10, and NIST triggers.
• Expanded EU AI Act coverage in README.md to include the official phased implementation timeline and clarified references to red teaming obligations.
• Added and linked newer NIST/OWASP/CSA resources in README.md and updated evidence/backing language where appropriate.
• Extended resources-validation.md with additional rows and Last Validated dates set to 2026-04-27 for NIST AI RMF + Playbook, SP 800-218A, SP 800-218 Rev.1, OWASP GenAI + Agentic Top 10, MITRE ATLAS, CSA Agentic AI Guide, and the EU AI Act implementation page.
• Updated CHANGELOG.md to summarize the README refresh and note the expansion of resources-validation.md.

Testing

• Ran documentation linting using markdownlint to validate formatting and it completed successfully.
• Executed a link-check pass with a linkcheck tool against updated external references and all checked links resolved successfully.
• No code-level unit tests were necessary because this is a documentation-only change.