Add nvim-tree, on the right, toggled with <leader>e

Netrw is disabled, as nvim-tree needs to own directory buffers so
`nvim .` opens the tree rather than the two fighting over it. That
removes :Ex, so the Obsidian guide has been updated to point at
<leader>e instead.

No auto-open autocmd: `nvim <path>` opens just the file, and the tree
is there on <leader>e when it is wanted. Placing it on the right also
flips the split direction, so opening a file from the tree puts it to
the left rather than displacing the tree.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

neovim/nvim-pack-lock.json

@@ -8,6 +8,10 @@
       "rev": "31d6fb2d618bca1482b9f274751ead5f03461408",
       "src": "https://github.com/lewis6991/gitsigns.nvim"
     },
+    "nvim-tree.lua": {
+      "rev": "b2aadda94b107480c48e548d6db51c6840b7b33c",
+      "src": "https://github.com/nvim-tree/nvim-tree.lua"
+    },
     "nvim-web-devicons": {
       "rev": "2ae6958df7ced50baac5035cec0c15799eedfbf7",
       "src": "https://github.com/nvim-tree/nvim-web-devicons"