docs: add CodeCompanion help file and Makefile doc downloader

- add Makefile target to download codecompanion.txt for both Unix and Windows
- include scripts/download_codecompanion.ps1 for Windows PowerShell support
- add full codecompanion.txt Neovim help documentation to doc/ directory

Author: jinzhongjia

Makefile

@@ -0,0 +1,33 @@
+.PHONY: doc
+
+OS := $(shell uname -s 2>/dev/null || echo Windows_NT)
+
+# Default output path for the downloaded doc
+DOC_OUT := doc/codecompanion.txt
+
+# Windows (PowerShell 7) target
+ifeq ($(OS),Windows_NT)
+# Prefer pwsh if available, fallback to powershell
+POWERSHELL := $(if $(shell where pwsh 2> NUL),pwsh,powershell)
+
+doc:
+	$(POWERSHELL) -NoProfile -ExecutionPolicy Bypass -File scripts/download_codecompanion.ps1 -OutFile $(DOC_OUT)
+
+else
+# Non-Windows target uses curl or wget
+CURL := $(shell command -v curl 2>/dev/null)
+WGET := $(shell command -v wget 2>/dev/null)
+URL := https://github.com/olimorris/codecompanion.nvim/raw/refs/heads/main/doc/codecompanion.txt
+
+doc:
+	@mkdir -p doc
+ifeq ($(CURL),)
+ifeq ($(WGET),)
+	@echo "Error: need curl or wget to download on non-Windows" && exit 1
+else
+	@$(WGET) -O $(DOC_OUT) $(URL)
+endif
+else
+	@$(CURL) -fsSL -o $(DOC_OUT) $(URL)
+endif
+endif