…-318] (infracost#3586)

The config-template parser (readFile/pathExists/isDir/matchPaths) and the
hosted-app file()/templatefile() guard both confined paths with a lexical
Rel/HasPrefix check plus a leaf-only Lstat/EvalSymlinks. An intermediate
in-repo directory symlink (evil -> /etc, then readFile "evil/passwd")
defeats both: the path is lexically clean and the leaf isn't a symlink, so
it passed - but the read followed the symlink out of the repo.

Add internal/security.IsPathAllowed as the single containment boundary: it
resolves symlinks anywhere in the path (leaf and intermediate) before a
segment-aware prefix compare against the resolved parent, with a
longest-existing-prefix fallback for not-yet-existent paths and a
process-wide resolve cache. Route the template parser and the Terraform
funcs guard through it and drop the old isSubdirectory/symlinkPath helpers.

Mirrors the recent fixes in the v2 config (#14) and parser (infracost#144) repos.