Add evidence-first DISHA Brain spine

[Tashima-Tarsh]

Summary

• Adds a typed evidence-first DISHA Brain graph with intake, evidence, context, reasoning, version routing, Yudh, Vyuha, NFU policy, approval, audit, and memory nodes.
• Formalizes six DISHA version modules, evidence classes, NFU enforcement, Vyuha formations, Yudh assessment, audit hash chain, geospatial/governance/HSE/development primitives, and demo payloads.
• Rewrites the README and adds canonical docs for architecture, product thesis, version ladder, Brain, agentic flow, NFU, evidence, trust, demos, Yudh/Vyuha, and roadmap.
• Adds focused tests and a GitHub Actions workflow for Brain lint, Bandit, and graph tests.

Validation

• python -m pytest tests/test_disha_brain_graph.py -q passes: 22 tests.
• python -m ruff check disha/brain tests passes.
• python -m bandit -q -r disha/brain passes.
• API smoke test passes for context.graph.invoke(GraphInput(...)).

Notes

• Existing web dependencies were not installed in this fresh clone, and no web code was changed.
• Claims about thenitishkr.in and the two DISHA Intelligence books remain marked [VERIFY REQUIRED] because source material is not present in the repository.