fix: harden devcontainer security controls

[pmembrey]

Summary

• restrict write-capable Claude automation to trusted actors and remove unused OIDC permissions
• harden devcontainer generation with feature allowlisting, structured mounts, variable-source rejection, build-path symlink checks, and security-opt validation
• harden template writes against symlink targets and pin bundled supply-chain inputs
• update README/template policy docs and add focused regression tests

Verification

• ./scripts/verify.sh

Note: cargo-deny still reports the existing duplicate-version warnings for TOML-related crates; advisories, bans, licenses, and sources pass.