nvim: render markdown in the buffer

render-markdown.nvim draws headings, code blocks, tables and checkboxes in
place, in the one buffer, and leaves the cursor line raw so editing is
unaffected; <leader>mt toggles it. The markdown and markdown_inline parsers
join the install list, which also gives noice's LSP hover rendering the
parsers it was already set up to use.

None of the colourschemes here define the @markup.heading.N.markdown captures
the plugin colours headings from, so every level rendered alike. A ramp fills
them in, only where the theme left the group empty, and again on each theme
switch. Checked with the real config: 26 marks on a test file, conceallevel 3,
H1-H3 in distinct bold colours.

AGENTS.md also stops listing nvim as having no machine-local hook: lua/local.lua
is one, gitignored and loaded by cfg-watch after the plugins.

From change request 12882c.

2 files changed, 74 insertions, 0 deletions

diff --git a/config/nvim/lua/plugins/lsp.lua b/config/nvim/lua/plugins/lsp.lua
index c3853b3..1470177 100644
--- a/config/nvim/lua/plugins/lsp.lua
+++ b/config/nvim/lua/plugins/lsp.lua
@@ -128,6 +128,7 @@ return {
             local TS = require("nvim-treesitter")
             local ensure_installed = {
                 "astro", "go", "html", "javascript", "lua", "python", "tsx", "typescript",
+                "markdown", "markdown_inline",
             }
             local installed = TS.get_installed("parsers")
             local to_install = vim.tbl_filter(function(lang)
diff --git a/config/nvim/lua/plugins/markdown.lua b/config/nvim/lua/plugins/markdown.lua
new file mode 100644
index 0000000..b298554
--- /dev/null
+++ b/config/nvim/lua/plugins/markdown.lua
@@ -0,0 +1,73 @@
+-- In-buffer markdown rendering: headings, code blocks, tables, callouts and
+-- checkboxes rendered in place, in the one buffer. No preview window or split.
+-- The cursor line keeps its raw syntax (the plugin's anti_conceal), so editing
+-- is unaffected. <leader>mt toggles rendering for the current buffer.
+return {
+    {
+        "MeanderingProgrammer/render-markdown.nvim",
+        ft = { "markdown" },
+        dependencies = { "nvim-tree/nvim-web-devicons" },
+        opts = {},
+        config = function(_, opts)
+            require("render-markdown").setup(opts)
+
+            -- render-markdown takes its per-level heading colours from the
+            -- treesitter captures @markup.heading.{1..6}.markdown. None of the
+            -- colourschemes under config/nvim/colors/ define treesitter groups
+            -- at all -- happy_hacking, onehalfdark and ayu were each checked --
+            -- so those captures resolve to nothing and every heading level
+            -- renders with no colour. Supply a ramp, but only where the theme
+            -- left the group undefined, so a theme that does define them keeps
+            -- its own. Re-applied on ColorScheme, since cfg-watch switches the
+            -- theme at runtime and `:colorscheme` can clear highlight groups.
+            local ramps = {
+                dark = {
+                    { "#F3F2CC", 230 },
+                    { "#FAD566", 221 },
+                    { "#B3EBBF", 157 },
+                    { "#81A2C7", 103 },
+                    { "#F77EBD", 211 },
+                    { "#fcb280", 179 },
+                },
+                light = {
+                    { "#2F2F2F", 236 },
+                    { "#8C6D1F", 136 },
+                    { "#3F6B3F", 22 },
+                    { "#3A5A8C", 24 },
+                    { "#8C3A6B", 89 },
+                    { "#8C5A2F", 130 },
+                },
+            }
+            local function set_heading_colors()
+                local ramp = ramps[vim.o.background] or ramps.dark
+                for level = 1, 6 do
+                    local name = "RenderMarkdownH" .. level
+                    -- Put render-markdown's own link back first, so a theme that
+                    -- does define the capture wins, and so this stays correct
+                    -- after a theme switch.
+                    vim.api.nvim_set_hl(0, name, { link = "@markup.heading." .. level .. ".markdown" })
+                    local hl = vim.api.nvim_get_hl(0, { name = name })
+                    if not (hl.fg or hl.bg or hl.bold or hl.italic or hl.underline) then
+                        vim.api.nvim_set_hl(0, name, {
+                            fg = ramp[level][1],
+                            ctermfg = ramp[level][2],
+                            bold = true,
+                        })
+                    end
+                end
+            end
+            set_heading_colors()
+            vim.api.nvim_create_autocmd("ColorScheme", { callback = set_heading_colors })
+
+            vim.api.nvim_create_autocmd("FileType", {
+                pattern = "markdown",
+                callback = function(event)
+                    vim.keymap.set("n", "<leader>mt", "<cmd>RenderMarkdown buf_toggle<cr>", {
+                        buffer = event.buf,
+                        desc = "Toggle markdown rendering",
+                    })
+                end,
+            })
+        end,
+    },
+}