Improve packaged build, skill ACL, and config compatibility

[Lixtt]

Summary

• Use the published a3s-common crate so standalone Code checkouts and Python wheel builds do not require a sibling ../../common checkout.
• Make skill allowed-tools handling compatible with omitted values, whitespace-only legacy values such as Read Write Edit Bash, and YAML list values.
• Parse ACL model token limits from flat model fields and nested limit blocks, including snake_case and camelCase aliases.

Why

These changes make packaged installs and benchmark/task catalogs work without relying on local build patches. They preserve explicit skill allowlists while avoiding accidental deny-all behavior for skills that omit allowed-tools, and they allow long-context OpenAI-compatible model deployments to express output/context limits in ACL config.

Validation

• cargo test -p a3s-code-core test_config_supports_acl_style_provider_labels
• cargo test -p a3s-code-core test_parse_legacy_whitespace_allowed_tools
• cargo test -p a3s-code-core test_parse_allowed_tools_yaml_list
• cargo test -p a3s-code-core test_skill_permission_policy_allows_when_unspecified
• cargo test -p a3s-code-core test_skill_permission_policy_accepts_legacy_allowed_tools
• cargo test -p a3s-code-core test_config_parses_acl_model_token_limits

[Lixtt]

Closed in favor of #34 with neutral branch and title naming.