docs: ratify gr2 UX model and workflow

[laynepenney]

Summary

• split the docs/rulebook content out of #556 so the ratified UX model can land independently
• add the gr2 UX manifesto, shared scratchpad design, stress matrix, and unit-local-first design correction
• codify the gr2 development workflow mantra: (design -> prototype -> verify)^n -> build -> assess -> repeat

What this lands

• docs/MANIFESTO-gr2-ux.md
• docs/PLAN-gr2-shared-scratchpads.md
• docs/ASSESS-gr2-shared-scratchpads-stress.md
• updates to docs/PLAN-gr2-needs-and-criteria.md
• updates to docs/PLAN-gr2-repo-maintenance-and-lanes.md

Ratified decisions

• gr2 is a multi-repo workspace router, not a git replacement
• lanes are the primary UX object
• unit-local and lane-local checkouts are the working surface
• shared repo cache is apply substrate / optimization, not the user-facing workspace model
• shared scratchpads are doc-first for MVP

Context

This is the first split out of #556 per review. Prototype code and harnesses will follow in separate PRs.

[laynepenney]

Scope is corrected now.

grip#557 is docs-only after rebasing and removing the prototype/code contamination from the branch. The diff against main is now limited to:

• docs/MANIFESTO-gr2-ux.md
• docs/PLAN-gr2-needs-and-criteria.md
• docs/PLAN-gr2-repo-maintenance-and-lanes.md
• docs/PLAN-gr2-shared-scratchpads.md
• docs/ASSESS-gr2-shared-scratchpads-stress.md
• docs/ASSESS-gr2-lanes-cross-mode-stress.md

I also added the new lane-model adversarial verification doc that pressures the design across all four operating modes:

• solo human
• single agent
• multi-agent
• mixed human + agent

That doc focuses on interruption recovery, current-context ambiguity, same-repo parallelism, mixed-mode lane conflicts, and machine-readable recovery/state surfaces. This should give us the same kind of hostile verification gate for the lane model that we already had for scratchpads.