Implement-with-design (FP-first)

Execute exactly one phase of a plan. Output is real production code plus an implementation report. The skill leaves the working tree on a clean series of atomic commits — one per acceptance criterion — and never amends.

Inputs and outputs

• Plan input: <cwd>/.plans/<feature>/plan.md — a phased plan file. Phases are demarcated by --- separators with ## Phase N: <Title> headings and - [ ] acceptance criteria.
• Design input (UI phases only): <cwd>/.plans/UI-RULES.md (the project's UI rulebook — aesthetic direction, typography, colour & theme, spatial & motion decisions, project-specific bans, AI Slop Test) and <cwd>/.plans/DESIGN.md (design tokens — colors, typography, rounded, spacing). Both are loaded during precondition step 3 — see step 3 for behaviour when either is missing.
• Code output: edits to project source.
• Report output: <cwd>/.plans/<feature>/implementations/with-design/{feat,fix,refactor}/<phase_number>.md — what was built, decisions made (FP choices, complexity, design decisions for UI), and any deviations from the plan. Subdirectory choice (feat / fix / refactor) follows the dominant intent of the phase: new behaviour → feat, bug-fix → fix, internal restructure with no behaviour change → refactor. If mixed, default to feat.

Modes

Two modes — default and TDD. Pick once at the start of the invocation; do not switch mid-phase.

• Default mode — write code, then run the narrowest automated check that proves the AC, then commit. One commit per AC.
• TDD mode — strict red → green → refactor per AC. Activated when the user's prompt contains "tdd", "use tdd", "tdd mode", or "test-first", or when (during preconditions) the user explicitly opts in. In TDD mode, each AC produces 2–3 commits ([red] failing test, [green] passing code, optional refactor). Full discipline in TDD.md.

If the prompt contains a near-trigger that doesn't match the list exactly (e.g. "write tests as you go", "test everything"), ask once with AskUserQuestion during preconditions; default to non-TDD if they decline to choose. Do not ask when no trigger phrase appeared — silence means default mode.