feat(conflict): detect and resolve inline merge conflict markers

Problem: when git hits a merge conflict, users stare at raw <<<<<<<
markers with broken treesitter and noisy LSP diagnostics. Existing
solutions (git-conflict.nvim) use their own highlighting rather than
integrating with diffs.nvim's color blending pipeline.

Solution: add conflict.lua module that detects <<<<<<</=======/>>>>>>>
markers (with diff3 ||||||| support), highlights ours/theirs/base
regions with blended DiffsConflict* highlight groups, provides
resolution keymaps (doo/dot/dob/don) and navigation (]x/[x),
suppresses diagnostics while markers are present, and auto-detaches
when all conflicts are resolved. Fires DiffsConflictResolved user
event on last resolution.

plugin/diffs.lua

@@ -30,6 +30,15 @@ vim.api.nvim_create_autocmd('BufReadCmd', {
   end,
 })
 
+vim.api.nvim_create_autocmd('BufReadPost', {
+  callback = function(args)
+    local conflict_config = require('diffs').get_conflict_config()
+    if conflict_config.enabled then
+      require('diffs.conflict').attach(args.buf, conflict_config)
+    end
+  end,
+})
+
 vim.api.nvim_create_autocmd('OptionSet', {
   pattern = 'diff',
   callback = function()