feat: add docfill mvp cli

docs/superpowers/plans/2026-06-18-docfill-mvp.md

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+# Docfill MVP Implementation Plan
+
+> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.
+
+**Goal:** Build a minimal Go CLI that fills `.docx` and `.xlsx` templates from Excel or SQLite rows.
+
+**Architecture:** A single internal package loads JSON config, reads rows from Excel or SQLite, renders placeholders into docx/xlsx files, and exposes `Run`. A tiny CLI parses `run --config`.
+
+**Tech Stack:** Go, `excelize` for `.xlsx`, `modernc.org/sqlite` for SQLite, standard `archive/zip` for `.docx`.
+
+---
+
+### Task 1: Tests First
+
+**Files:**
+- Create: `internal/docfill/docfill_test.go`
+
+- [ ] Write tests for Excel row reading, SQLite row reading, xlsx rendering, docx rendering, and full config execution.
+- [ ] Run `go test ./...` and confirm it fails because the package implementation does not exist yet.
+
+### Task 2: Minimal Library
+
+**Files:**
+- Create: `internal/docfill/docfill.go`
+
+- [ ] Implement config parsing.
+- [ ] Implement Excel and SQLite row readers.
+- [ ] Implement placeholder replacement.
+- [ ] Implement xlsx and docx rendering.
+- [ ] Implement `Run(configPath string) error`.
+
+### Task 3: CLI
+
+**Files:**
+- Create: `cmd/docfill/main.go`
+
+- [ ] Parse `docfill run --config task.json`.
+- [ ] Print clear errors to stderr.
+- [ ] Exit non-zero on failure.
+
+### Task 4: Verify
+
+- [ ] Run `go test ./...`.
+- [ ] Run `go build ./cmd/docfill`.
+- [ ] Keep README usage minimal.