feat: add initial skill authoring workspace

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

skills/office-export-xlsx/references/export-flow.md

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+# Export Flow
+
+This skill assumes an upstream structured artifact already exists.
+
+## Input Contract
+
+Required keys:
+
+- `sheet_name`
+- `columns`
+- `rows`
+
+Recommended keys:
+
+- `source`
+- `partial`
+- `notes`
+
+## Export Sequence
+
+1. Confirm the task is export-only.
+2. Validate that `sheet_name` is non-empty.
+3. Validate that `columns` is a non-empty list.
+4. Validate that `rows` is a list of row arrays aligned to `columns`.
+5. Call `openxml_office` with:
+   - template path
+   - output `.xlsx` path
+   - `sheet_name`
+   - `columns`
+   - `rows`
+6. Return the generated workbook path and any partial-data warning.
+
+## Output Contract
+
+Return:
+
+- output file path
+- sheet name
+- row count
+- whether the source artifact was partial