feat(resolve): add resolve command and log integration (#125)

Add first-class conflict resolution support to jj.nvim.

   - add `jj.cmd.resolve` to run `jj resolve` in a floating terminal or via an
     external tool
   - add `:J resolve` with support for `-r/--revision`, `--tool`,
     `--external/--ext`, and positional filesets
   - integrate resolve into the log buffer with the `gr` mapping and optional
     `resolve_strategies` picker
   - add `utils.is_change_conflicted()` and avoid opening resolve when the
     selected revision has no conflicts
   - document resolve usage, config, and keymaps in the README
   - add tests for resolve arg parsing and conflict detection helpers

lua/jj/utils.lua

@@ -644,6 +644,24 @@ function M.is_change_empty(revset)
 	return vim.trim(output) == "true"
 end
 
+--- Check if a given revset has conflicts
+--- @param revset string The revset to check
+--- @return boolean True if the revset has conflicts, false otherwise
+function M.is_change_conflicted(revset)
+	local output, success = runner.execute_command(
+		string.format("jj log --no-graph -r %s -T 'conflict' --quiet", vim.fn.shellescape(revset)),
+		"Error checking if revset has conflicts",
+		nil,
+		true
+	)
+
+	if not success or not output then
+		return false
+	end
+
+	return vim.trim(output) == "true"
+end
+
 --- Build describe text for a given revision
 --- @param revset? string The revision to describe (default: @)
 --- @return string[]|nil