Fix nvim transparency to preserve foreground colors (#4844)

* Fix nvim transparency to preserve foreground colors

The old approach used `vim.api.nvim_set_hl(0, name, { bg = "none" })`
which replaces the entire highlight definition, wiping out foreground
and other attributes. This breaks snacks.nvim's GitHub integration
(gh_pr/gh_issue pickers) which reads the foreground color from Normal
and NormalFloat at module load time, gets nil, and crashes in
Snacks.util.blend().

The fix uses nvim_get_hl to fetch existing attributes first, removes
only bg, then sets the highlight back — preserving all other attributes.

* Drop fallback that reintroduces the highlight-wiping bug

If nvim_get_hl fails for a group, skip it rather than falling back to
the old { bg = "none" } approach that clears all attributes.

migrations/1772389838.sh

@@ -0,0 +1,67 @@
+echo "Fix nvim transparency to preserve highlight foreground colors"
+
+TRANSPARENCY_FILE="$HOME/.config/nvim/plugin/after/transparency.lua"
+
+if [[ -f "$TRANSPARENCY_FILE" ]]; then
+  cat > "$TRANSPARENCY_FILE" << 'EOF'
+-- Make highlight groups transparent while preserving their other attributes
+local function make_transparent(name)
+	local ok, hl = pcall(vim.api.nvim_get_hl, 0, { name = name, link = false })
+	if ok then
+		hl.bg = nil
+		vim.api.nvim_set_hl(0, name, hl)
+	end
+end
+
+local groups = {
+	-- transparent background
+	"Normal",
+	"NormalFloat",
+	"FloatBorder",
+	"Pmenu",
+	"Terminal",
+	"EndOfBuffer",
+	"FoldColumn",
+	"Folded",
+	"SignColumn",
+	"LineNr",
+	"CursorLineNr",
+	"NormalNC",
+	"WhichKeyFloat",
+	"TelescopeBorder",
+	"TelescopeNormal",
+	"TelescopePromptBorder",
+	"TelescopePromptTitle",
+	-- neotree
+	"NeoTreeNormal",
+	"NeoTreeNormalNC",
+	"NeoTreeVertSplit",
+	"NeoTreeWinSeparator",
+	"NeoTreeEndOfBuffer",
+	-- nvim-tree
+	"NvimTreeNormal",
+	"NvimTreeVertSplit",
+	"NvimTreeEndOfBuffer",
+	-- notify
+	"NotifyINFOBody",
+	"NotifyERRORBody",
+	"NotifyWARNBody",
+	"NotifyTRACEBody",
+	"NotifyDEBUGBody",
+	"NotifyINFOTitle",
+	"NotifyERRORTitle",
+	"NotifyWARNTitle",
+	"NotifyTRACETitle",
+	"NotifyDEBUGTitle",
+	"NotifyINFOBorder",
+	"NotifyERRORBorder",
+	"NotifyWARNBorder",
+	"NotifyTRACEBorder",
+	"NotifyDEBUGBorder",
+}
+
+for _, name in ipairs(groups) do
+	make_transparent(name)
+end
+EOF
+fi