feat: add docfill mvp cli

docs/superpowers/specs/2026-06-18-docfill-mvp-design.md

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+# Docfill MVP Design
+
+## Goal
+
+Build the smallest local Go command-line tool that a digital employee can call to fill Word or Excel templates from Excel or SQLite data.
+
+## MVP Scope
+
+- Input data: `.xlsx` table or SQLite query result.
+- Template files: `.docx` or `.xlsx`.
+- Placeholder syntax: `{{column_name}}`.
+- Output: one generated file per input row.
+- Invocation: `docfill run --config task.json`.
+
+## Explicit Non-Goals
+
+- No web UI.
+- No permission system.
+- No workflow engine.
+- No template designer.
+- No direct business-system integration.
+- No AI guessing of fields.
+- No complex Word layout manipulation.
+
+## Config Shape
+
+```json
+{
+  "data": {
+    "type": "excel",
+    "path": "./data.xlsx",
+    "sheet": "Sheet1"
+  },
+  "template": {
+    "path": "./template.docx"
+  },
+  "output": {
+    "path": "./out/report_{{name}}_{{_row}}.docx"
+  }
+}
+```
+
+For SQLite, `data` uses `type: "sqlite"` and `query`.
+
+## Constraints
+
+- Keep the implementation boring and inspectable.
+- Infer template behavior from file extension.
+- Treat Excel first row or SQL column names as placeholder keys.
+- If a Word placeholder is split across Word XML runs, MVP may not replace it; users should type placeholders as plain contiguous text.