fix: increase training budget to 16K chars and rewrite docs as harness customization guide

Training is not a CLAUDE.md replacement — it's the mechanism by which users
customize the data engineering harness for their specific project. The agent
works WITH the user to discover what it needs to know, rather than requiring
users to write perfect static instructions.

Changes:
- Increase TRAINING_BUDGET from 6000 to 16000 chars (removes the #1 criticism
  from user simulations — budget was worse than unlimited CLAUDE.md)
- Complete docs rewrite with correct positioning:
  - "Customizing Your AI Teammate" framing (not "Training Your AI Teammate")
  - Research-backed "why" section (40-70% knowledge omission, guided discovery)
  - Clear comparison table: training vs CLAUDE.md (complementary, not competing)
  - 6 real-world scenarios including Databricks, Salesforce quirks, cost spikes
  - Honest limitations section (not a linter, not an audit trail, not automatic)

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

Author: anandgupta42