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/absolute/plugins/snacks.lua

@@ -0,0 +1,12 @@
+return {
+	"folke/snacks.nvim",
+	priority = 1000,
+	lazy = false,
+	---@type snacks.Config
+	opts = {
+		bigfile = { enabled = true },
+		quickfile = { enabled = true },
+		-- input, notifier added in Phase 2
+		-- picker added in Phase 3
+	},
+}