Consolidate root-level documentation

[avrabe]

Goal

Clean up the 48+ markdown files at the project root by consolidating and organizing them.

Tasks

• Move all session notes to docs/sessions/ or archive
• Consolidate Bazel docs → docs/build-systems/BAZEL.md
• Consolidate Sail docs → docs/research/SAIL_INTEGRATION.md
• Consolidate validation docs → docs/validation/STATUS.md
• Keep only 5 files at root: README, ARCHITECTURE, CHANGELOG, LICENSE, CONTRIBUTING
• Update README.md as single entry point with clear navigation

Acceptance Criteria

• ✅ ≤5 markdown files at project root
• ✅ All docs accessible via README navigation tree
• ✅ No duplicate/overlapping content

Files to Organize

SESSION_*.md → docs/sessions/
BAZEL_*.md → docs/build-systems/
SAIL_*.md → docs/research/
VALIDATION_*.md → docs/validation/
PHASE*.md → docs/status/