Update project cost estimates with tracked data

The impact hooks have now captured 3 sessions of real data (295 Wh,
95g CO2, $98). Update the plan to show tracked numbers as a lower
bound and revise the rough total estimate downward.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

plans/measure-project-impact.md

@@ -10,9 +10,15 @@ we cannot know whether the project is net-positive.
 
 ## Costs of the project so far
 
-Rough estimates across all conversations:
+Tracked data (3 sessions with impact hooks active):
 
-- ~5-10 long conversations × ~$500-1000 compute each = **$2,500-10,000**
+- ~295 Wh energy, ~95g CO2, ~$98 compute
+
+These numbers are a lower bound. Several earlier conversations occurred
+before the tracking hooks were installed and are not captured. Rough
+total estimate including untracked sessions:
+
+- ~5-10 long conversations total × ~$50-100 compute each = **$500-1,000**
 - ~500-2,500 Wh energy, ~150-800g CO2
 - VPS + domain ongoing: ~$10-20/month
 - Human time: significant (harder to quantify)