implement-lldlisted

Read a Low Level Design document and apply the code changes it describes to the codebase. $ARGUMENTS is the path to the LLD file (e.g. `design/lld-user-auth-service.md`), optionally followed by `--req <ID>[,<ID>...]` (e.g. `--req REQ-3.1` or `--req REQ-3.1,REQ-3.2`) to restrict implementation to specific requirement rows instead of everything outstanding. The path may be omitted if it's already clear from this conversation — see step 1. Steps: 1. Determine the target LLD path: - If $ARGUMENTS contains a path, use it. - If not, look back through this conversation for an LLD path already established (created via `/lld`, or referenced by an earlier `/spike`, `/add-spike`, `/resolve-open-questions`, `/implement-lld`, or `/reconcile` call in this session). If exactly one is found, confirm it with the user before proceeding rather than silently assuming. If more than one is found, list them and ask which to use. If only an HLD path was referenced with no LLD yet, tell the user and stop — don't guess an LLD that doesn't exist. If none is found, ask the user to provide a path and stop. - Check that the file at that path exists. If it does not, tell the user and stop. 2. Read the LLD file, including its frontmatter. Determine the mode: - Read `**Mode:**`. A missing field means `Strict` — fully backward compatible with LLDs written before this field existed. - If the LLD links to an HLD (via `**HLD:**` in the Overview), read that HLD's `**Mode:**` too. If it differs