feat(nvim): phase 1 — triage and abandoned-plugin cleanup

Fix keymap collision on <leader>j/<leader>k (now resize-only;
location-list moved to ]l/[l). Migrate diagnostic signs from
deprecated vim.fn.sign_define to vim.diagnostic.config. Drop
orphan after/nvim-tree.lua and the bogus require("../current-theme").
Add foundation snacks.nvim spec. Remove playground, vim-maximizer,
poet-v, git-blame, render-markdown, and all five AI plugin specs.
Enable gitsigns current_line_blame.

lua/current-theme.lua

@@ -1 +1 @@
-vim.cmd("colorscheme catppuccin-macchiato")
+vim.cmd("colorscheme tokyonight-night")