[codex] Finalize mindspace spec

[dudash]

Summary

• Add a self-contained docs/mindspace/ section for the optional Mindspace workspace layer.
• Capture the canonical Mindspace reference contract, boundaries, tips, user value, and TUI navigation model.
• Update the docs portal, agent CLI contract, and Mindspace PRD to point at the new Mindspace section and canonical mdm mindspace ... namespace.
• Align Linear follow-up issues so implementation work points at docs/mindspace/REFERENCE.md and the navigation/value docs.

Why

Mindspace should be optional and self-contained: mdm and mdmind can support it, but workspace-specific concepts should not bleed into core single-map behavior. This PR makes that boundary explicit while preserving the planned value: local-first workspace understanding, deterministic scan/lint/context contracts, and reviewable agent-safe maintenance.

Validation

• git diff --check
• Searched docs for stale docs/reference/MINDSPACE.md links and stale mdm agent session/review examples.
• Docs-only change; Rust tests were not run.