fix(files): allow symlinked storage roots

[Qyra-One]

Summary

• fix safeJoin to canonicalize the area root before comparing it with EvalSymlinks output
• keep the lexical containment check unchanged so traversal is still rejected before any symlink handling
• add regression coverage for a symlinked/canonicalized area root and for a child symlink that escapes the area root

Fixes #62.

Test plan

go test ./internal/files
go test ./...