feat: add docfill mvp cli

docs/digital-employee-runbook.md

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+# Digital Employee Runbook
+
+This file is the minimal operating contract for calling `docfill` from a digital employee, scheduler, or shell script.
+
+## Inputs
+
+Each run needs:
+
+- A `task.json` config file.
+- A data source: `.xlsx` or SQLite `.db`.
+- A template: `.docx` or `.xlsx`.
+- An output path pattern in `task.json`.
+
+Paths inside `task.json` are resolved from the config file directory.
+
+## Command
+
+```bash
+docfill run -c /path/to/task.json --var date=2026-06-18
+```
+
+Use repeated `--var key=value` arguments for runtime values:
+
+```bash
+docfill run -c task.json --var date=2026-06-18 --var batch=早班
+```
+
+CLI variables override `vars` in `task.json`. Row data overrides both.
+
+## Success
+
+Exit code `0`.
+
+Stdout contains generated files:
+
+```text
+generated 2 file(s)
+- /path/to/out/report_张三.xlsx
+- /path/to/out/report_李四.xlsx
+```
+
+## Failure
+
+Exit code `1` means the task ran but failed.
+
+Exit code `2` means the command line was invalid.
+
+Errors are written to stderr with the `docfill:` prefix:
+
+```text
+docfill: render row 1: missing template field "done"
+```
+
+## Common Errors
+
+`missing template field`
+
+The template contains a `{{field}}` that is not present in data, `vars`, or CLI `--var`.
+
+`output already exists`
+
+The output file exists and `output.overwrite` is not `true`.
+
+`missing sqlite params field`
+
+A SQLite query parameter such as `{{date}}` was not provided.
+
+`unresolved placeholder`
+
+The rendered document still contains `{{field}}`. Check the source data value and the template.
+
+## Build
+
+```bash
+bash scripts/build.sh
+```
+
+The local build output is:
+
+```text
+dist/docfill-darwin-arm64
+```
+
+## Smoke Test
+
+```bash
+bash scripts/smoke.sh
+```
+
+Smoke test builds a temporary binary, generates temporary example files, runs both example configs, and checks generated outputs.