feat(nvim): conceal wiki-link ids to styled name hyperlinks (§8.4)

`conceal.lua` hides the `[[id|` prefix and `]]` suffix with conceal
extmarks (refreshed on edit), leaving the label as a styled `HephLink`;
`conceallevel=2` + empty `concealcursor` reveal the raw `[[id|Name]]` on
the cursor's line so it stays editable. The `[[` picker now inserts the
labelled `[[id|Name]]` form (readable + conceal-ready; collapses to bare
on save). e2e asserts the conceal extmarks + conceallevel.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

heph.nvim/lua/heph/conceal.lua

@@ -0,0 +1,66 @@
+--- Conceal `[[NODEID|Name]]` links down to a styled "Name" hyperlink in node
+--- buffers (tech-spec §8.4). The node id is structural noise to a reader, so we
+--- hide the `[[id|` prefix and the `]]` suffix with conceal extmarks, leaving
+--- the label visible and highlighted. `conceallevel=2` + an empty
+--- `concealcursor` reveal the raw `[[id|Name]]` on the line the cursor is on, so
+--- it stays directly editable. A bare `[[id]]` (briefly, before save→reload
+--- canonicalises it) just hides its brackets.
+
+local M = {}
+
+local ns = vim.api.nvim_create_namespace("heph_link_conceal")
+
+--- Recompute conceal extmarks for every `[[…]]` span in `buf`.
+function M.refresh(buf)
+  if not vim.api.nvim_buf_is_valid(buf) then
+    return
+  end
+  vim.api.nvim_buf_clear_namespace(buf, ns, 0, -1)
+  local lines = vim.api.nvim_buf_get_lines(buf, 0, -1, false)
+  for lnum, line in ipairs(lines) do
+    local row = lnum - 1
+    local from = 1
+    while true do
+      local s = line:find("[[", from, true) -- 1-based byte of first `[`
+      if not s then
+        break
+      end
+      local e = line:find("]]", s + 2, true) -- 1-based byte of `]]`
+      if not e then
+        break
+      end
+      local inner = line:sub(s + 2, e - 1)
+      local pipe = inner:find("|", 1, true)
+      -- Byte columns are 0-based for extmarks; end_col is exclusive.
+      local open_end -- exclusive end of the hidden prefix (`[[` or `[[id|`)
+      if pipe then
+        open_end = (s + 1 + pipe) -- 0-based col just past the `|`
+      else
+        open_end = s + 1 -- 0-based col just past `[[`
+      end
+      -- Hide the prefix, highlight the visible label, hide the closing `]]`.
+      vim.api.nvim_buf_set_extmark(buf, ns, row, s - 1, { end_col = open_end, conceal = "" })
+      vim.api.nvim_buf_set_extmark(buf, ns, row, open_end, { end_col = e - 1, hl_group = "HephLink" })
+      vim.api.nvim_buf_set_extmark(buf, ns, row, e - 1, { end_col = e + 1, conceal = "" })
+      from = e + 2
+    end
+  end
+end
+
+--- Enable link conceal for the current window + `buf`: define the highlight,
+--- set the window conceal options, refresh now, and refresh on edits.
+function M.attach(buf)
+  vim.api.nvim_set_hl(0, "HephLink", { link = "Underlined", default = true })
+  vim.wo.conceallevel = 2
+  vim.wo.concealcursor = "" -- reveal the raw link on the cursor's line
+  M.refresh(buf)
+  vim.api.nvim_create_autocmd({ "TextChanged", "TextChangedI" }, {
+    buffer = buf,
+    callback = function()
+      M.refresh(buf)
+    end,
+    desc = "heph: refresh [[link]] conceal",
+  })
+end
+
+return M

heph.nvim/lua/heph/link.lua

@@ -94,8 +94,16 @@ function M.insert()
       if not choice then
         return
       end
-      local id = choice.__create and rpc.call("node.create", { kind = "doc", title = choice.title }).id or choice.id
-      vim.api.nvim_put({ "[[" .. id .. "]]" }, "c", true, true)
+      local id, title
+      if choice.__create then
+        local node = rpc.call("node.create", { kind = "doc", title = choice.title })
+        id, title = node.id, node.title
+      else
+        id, title = choice.id, choice.title
+      end
+      -- Insert the labelled form `[[id|Name]]` (readable + conceal-ready); it
+      -- collapses to the canonical bare `[[id]]` on save (§8.4).
+      vim.api.nvim_put({ "[[" .. id .. "|" .. title .. "]]" }, "c", true, true)
     end)
   end)
 end

heph.nvim/lua/heph/node.lua

@@ -34,6 +34,7 @@ function M.read(buf, uri)
   vim.bo[buf].fileformat = "unix"
   vim.bo[buf].modified = false
   require("heph.link").attach(buf)
+  require("heph.conceal").attach(buf)
 end
 
 --- `BufWriteCmd` handler: route frontmatter edits to RPCs, then save the body.