docs: document cheap main model and Board posture

[bearmug]

Summary

Closes #411.

• Document that Pi owns the active main model and Pi-Rogue never switches it.
• Recommend an explicitly selected cheap/fast DeepSeek Flash profile while warning that provider IDs, prices, limits, and availability change.
• Document Board watcher modes, optional Head escalation, and the role separation between main model, Advisor, Board, and Head.
• Show the active Pi model in /pi-rogue-advisor status without mutating configuration.
• Add documentation/runtime consistency tests.

Research basis

• DeepSeek official pricing/model documentation: https://api-docs.deepseek.com/quick_start/pricing-details-usd
• Pi's authenticated /list-models registry is the runtime authority for available provider/model IDs.

Validation

• npm test -- --run packages/advisor/src/extension.test.ts packages/advisor/src/completions.test.ts packages/bundle/src/extension.test.ts — 31 tests passed
• npm run check — passed
• git diff --check — passed

Stack

Base: PR #413 / issue #410. This is the documentation and runtime-truth capstone of the posture stack.