implement-planlisted

## Objective Execute a reviewed plan through its implementation steps. Each step gets a dedicated implementer agent, followed by a step hardener that verifies completeness and fixes emergent issues. After all steps, a standards review and final review close the loop. ## Context The plan to implement is given as `$ARGUMENTS`. It may be a direct path, a natural-language reference (e.g. "the plan we just wrote"), or empty. ## 0. Validate and resolve the plan path Resolve `$ARGUMENTS` to a concrete plan file: 1. If `$ARGUMENTS` is a path to an existing file, use it directly. 2. If `$ARGUMENTS` is empty or is a phrase rather than a path (e.g. "the plan we just wrote", "the latest one"), interpret it and locate the intended plan among `plans/*.md` (for "the latest" or "the one we just wrote", the most recently modified `plans/*.md` file). This is a guess that launches a full implementation run, so state the resolved path back to the user and confirm before proceeding. 3. If it cannot be resolved to a single file (`$ARGUMENTS` looks like a path but the file does not exist, `plans/` is empty or absent, nothing matches, or several plausibly match), use AskUserQuestion to ask: "Which plan file should I implement?" with a hint to provide the path. From this point on, `$PLAN_PATH` refers to this resolved plan file. Use `$PLAN_PATH`, not the raw `$ARGUMENTS`, everywhere below. Read the plan file at `$PLAN_PATH`. If it doesn't exist, error and stop. Check that the plan has an `##