docs: expand security documentation in README and SECURITY.md

[max-rattray-aws]

Summary

Improves security guidance for users and vulnerability reporters by expanding both README.md and SECURITY.md.

SECURITY.md

• Adds 6 vulnerability classes to the in-scope definition: parser DoS, credential leakage, symlink/path traversal, URL parsing differentials, credential injection on redirects, MCP session isolation
• Adds Security Architecture section describing the Kernel trait boundary and its four enforcement layers (filesystem isolation, SSRF guard, credential injection, no-syscall)
• Adds explicit Out of Scope section aligned with the documented threat model

README.md

• Adds security disclaimer callout at the top of the Security Model section clarifying that Strands Shell is a mediation layer, not a hardened sandbox
• Adds Secure Defaults subsection with least-privilege guidance (copy > direct, narrow binds, explicit URL allowlists, timeouts, output limits)
• Adds warning about mode: "direct" mounts in the Configuration section

Testing

Documentation-only change.