docs: reorganize docs landscape per master plan

docs/README.md

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+# CORE Documentation Index
+
+This is the central index for all documentation in the CORE project.
+
+## Directories
+- `adr/` - Architecture decision records
+- `analysis/` - Deep dives and master plans
+- `architecture/` - Architecture docs
+- `audit/` - Audit reports
+- `benchmarks/` - Benchmark evidence
+- `briefs/` - Briefs
+- `curriculum/` - Teaching/learning order
+- `evals/` - Eval docs
+- `examples/` - Usage examples
+- `handoffs/` - Handoff docs
+- `implementation/` - Implementation guides
+- `issues/` - Detailed issue analyses
+- `lab/` - Experimental content (Not ratified)
+- `outreach/` - External comms
+- `plans/` - Plans
+- `research/` - Research notes
+- `sessions/` - Session logs
+- `specs/` - Specifications
+- `workbench/` - Workbench docs
+- `zig/` - Zig language docs
+- `agents/` - Agent-specific operational guides
+
+> **Note regarding `grokbuild/`**: Agent-specific operational guide for Grok 4.3 + Grok Build. Not architecture documentation. Candidate for relocation to `docs/agents/grok/`.