build-milestonelisted

# Build a milestone Execute ONE milestone of a learning project end-to-end. Each milestone ships real working software (backend **and** frontend/output) and teaches one concept. All product decisions are already locked in the milestone spec — do not re-open them. ## When this triggers The user says "do Mx", "build milestone X", "ship Mx", or "continue the project" for a project under `projects/*` that has a `ROADMAP.md` + `specs/`. ## Preconditions to check first 1. Identify the active project (obey the user if they name one; otherwise the one in progress). 2. Open `ROADMAP.md` → confirm the target milestone and read the **global locked decisions**. 3. Open `specs/Mx-*.md` → read its **locked decisions**, **entry criteria**, scope, data-model delta, API, UI, acceptance criteria, and task outline. 4. Verify the milestone's **entry criteria** are met (the prior milestone's deliverables exist). If not, tell the user which milestone must be built first and stop. ## Procedure 1. **Announce** which milestone you're building and its concept. 2. **Expand the plan (just-in-time).** If `plans/Mx-*.md` doesn't exist yet, turn the spec's task outline into a granular, task-by-task plan that references the **actual current code** (real file paths, real signatures from prior milestones). Save it to `plans/Mx-<name>.md`. Commit it before writing feature code. 3. **Build task-by-task** (test-driven where logic exists): - Follow the project's locked stack (see `R