docs: make DeskGraph reviewer-first without losing technical detail

[masddffee]

Summary

• rewrite the root README for a 60-second reviewer path: positioning, architecture, current status, evidence, quick start, safety, and limitations
• add docs/README.md as the documentation map and make docs/planning/IMPLEMENTATION_STATUS.md the explicit canonical detailed status ledger
• move command-level/development detail into docs/DEVELOPMENT_REFERENCE.md instead of deleting it

Design goals

• preserve technical credibility and explicit non-claims
• avoid a second competing implementation-status document
• make the root README useful to OSS reviewers before exposing milestone/ADR-level detail
• keep detailed scan, extraction, OCR, search, graph, watch, and organization-preview workflows discoverable

Verification performed

• compared main to this branch: only three documentation files change
• re-fetched all three rendered source files from the branch
• cross-checked key claims against the existing implementation status, MCP documentation, benchmark, architecture, security policy, and repository structure
• preserved the pre-release, Windows/runtime, vector/hybrid, Watch, file-action, packaging, and 8 GB evidence boundaries

No product code or runtime behavior is changed.