feat: display CO2 in kg when >= 1000g for readability

Switch CO2 emissions display from always-grams to adaptive
kg/g notation: values >= 1 000 g are shown as X.XX kg.

- vscode-extension webview environmental panel: add formatCo2Grams()
  helper that wraps smartFixed(); use it for the 4 period cards
- cli environmental command: add formatCo2() helper; use it for the
  per-period CO2 row and the methodology tree-absorption line;
  remove unused mt import

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: rajbos

cli/src/commands/environmental.ts

@@ -3,7 +3,7 @@
  */
 import { Command } from 'commander';
 import chalk from 'chalk';
-import { discoverSessionFiles, calculateDetailedStats, fmt, formatTokens, ENVIRONMENTAL } from '../helpers';
+import { discoverSessionFiles, calculateDetailedStats, formatTokens, ENVIRONMENTAL } from '../helpers';
 import type { PeriodStats } from '../../../vscode-extension/src/types';
 
 export const environmentalCommand = new Command('environmental')
@@ -38,7 +38,7 @@ export const environmentalCommand = new Command('environmental')
 		console.log(chalk.dim('Methodology: Estimates based on industry averages for AI inference'));
 		console.log(chalk.dim(`  CO₂: ${ENVIRONMENTAL.CO2_PER_1K_TOKENS} gCO₂e per 1K tokens`));
 		console.log(chalk.dim(`  Water: ${ENVIRONMENTAL.WATER_USAGE_PER_1K_TOKENS} L per 1K tokens`));
-		console.log(chalk.dim(`  Tree absorption: ${fmt(ENVIRONMENTAL.CO2_ABSORPTION_PER_TREE_PER_YEAR)} g CO₂/year\n`));
+		console.log(chalk.dim(`  Tree absorption: ${formatCo2(ENVIRONMENTAL.CO2_ABSORPTION_PER_TREE_PER_YEAR)} CO₂/year\n`));
 
 		for (const period of periods) {
 			printEnvironmentalStats(period.label, period.emoji, period.stats);
@@ -75,6 +75,14 @@ export const environmentalCommand = new Command('environmental')
 		console.log(chalk.dim(`Last updated: ${stats.lastUpdated.toLocaleString()}\n`));
 	});
 
+/** Format CO₂ in grams, switching to kg notation when ≥ 1 000 g */
+function formatCo2(grams: number): string {
+	if (grams >= 1000) {
+		return `${(grams / 1000).toFixed(2)} kgCO₂e`;
+	}
+	return `${grams.toFixed(3)} gCO₂e`;
+}
+
 function printEnvironmentalStats(label: string, emoji: string, stats: PeriodStats): void {
 	console.log(chalk.bold(`${emoji} ${label}`));
 	console.log(chalk.dim('─'.repeat(55)));
@@ -86,7 +94,7 @@ function printEnvironmentalStats(label: string, emoji: string, stats: PeriodStat
 	}
 
 	console.log(`  Tokens used:          ${chalk.bold.yellow(formatTokens(stats.tokens))}`);
-	console.log(`  CO₂ emissions:        ${chalk.bold(stats.co2.toFixed(3))} gCO₂e`);
+	console.log(`  CO₂ emissions:        ${chalk.bold(formatCo2(stats.co2))}`);
 	console.log(`  Water usage:          ${chalk.bold(stats.waterUsage.toFixed(3))} liters`);
 
 	if (stats.treesEquivalent > 0) {

vscode-extension/src/webview/environmental/main.ts

@@ -79,6 +79,14 @@ function smartFixed(value: number): string {
 	return formatFixed(Math.round(value), 0);
 }
 
+/** Format CO₂ in grams, switching to kg notation when ≥ 1 000 g */
+function formatCo2Grams(grams: number): string {
+	if (grams >= 1000) {
+		return `${smartFixed(grams / 1000)} kg`;
+	}
+	return `${smartFixed(grams)} g`;
+}
+
 type AnalogyItem = { icon: string; text: string };
 
 const co2AnalogyItems = (grams: number): AnalogyItem[] => [
@@ -188,10 +196,10 @@ function buildImpactCards(
 		],
 		// CO₂ card
 		[
-			['📅 Today', `${smartFixed(stats.today.co2)} g`, co2AnalogyItems(stats.today.co2)],
-			['📈 Last 30 Days', `${smartFixed(stats.last30Days.co2)} g`, co2AnalogyItems(stats.last30Days.co2)],
-			['📆 Previous Month', `${smartFixed(stats.lastMonth.co2)} g`, co2AnalogyItems(stats.lastMonth.co2)],
-			['🌍 Projected Year', `${smartFixed(projectedCo2)} g`, co2AnalogyItems(projectedCo2)],
+			['📅 Today', formatCo2Grams(stats.today.co2), co2AnalogyItems(stats.today.co2)],
+			['📈 Last 30 Days', formatCo2Grams(stats.last30Days.co2), co2AnalogyItems(stats.last30Days.co2)],
+			['📆 Previous Month', formatCo2Grams(stats.lastMonth.co2), co2AnalogyItems(stats.lastMonth.co2)],
+			['🌍 Projected Year', formatCo2Grams(projectedCo2), co2AnalogyItems(projectedCo2)],
 		],
 		// Water card
 		[