docs: document MCP 2026-07-28 dual-stack migration

[jackbatzner]

Summary

Update MCP docs for issue #2597 after implementation lands so AGT documents stateless 2026-07-28 as the preferred behavior and 2025-11-25 as compatibility.

Scope

• Update docs\tutorials\27-mcp-scan-cli.md.
• Update related MCP trust/security docs found during implementation.
• Stop describing Mcp-Session-Id as the normal scanner lifecycle.
• Distinguish AGT application-level auth/state helpers from MCP protocol sessions.
• Update ADR 0027 status only if maintainers want the implementation PR to carry that change.

Testing

• Run relevant docs checks for edited files.
• Verify commands, links, and lifecycle descriptions match shipped behavior.

Parent: #2597