docs(docs): Add Unified Thinking Model architecture

[shettydev]

Description

This PR introduces the Unified Thinking Model documentation, detailing the architectural solution for the "Endless Dialogue Problem" and the "Two-World Problem" (Chat vs Canvas). It proposes a graph-based approach to thinking sessions where conversations have distinct resolution states.

Changes

• Added packages/mukti-docs/planning/unified-thinking-model.md
• Defined the Branch Resolution Model for node and session states (Exploring, Converging, Resolved).
• Outlined the Convergence Protocol to detect user understanding and guide them to closure.
• Detailed the Chat-Canvas Convergence strategy, treating every conversation as a linear traversal of a thought graph.
• Proposed schema extensions for NodeDialogue and CanvasSession.

Testing

• N/A (Documentation only)

Screenshots (if applicable)

• N/A

Checklist

• Code follows project style guidelines
• Tests added/updated
• Documentation updated (if needed)
• No breaking changes (or documented if any)

Additional Notes

This document serves as the roadmap for the implementation of the new Socratic Engine mechanics, including state-aware prompting and the synthesis phase.