fix(ppstack): restore model-diverse review roles

[peterpme]

Why

PR #58 replaced task-specific model routing with repeated calls to one generic role. Reflect still named three different review lenses, but all three ran on the same Grok reviewer. How critique, Interrogate, and Arena retained language about model diversity while their workflows also repeated one role.

The role name must select the model. A failed lane must remain unavailable instead of silently changing its model or purpose.

Scope

• Add fresh read-only reviewer-sol, reviewer-grok, reviewer-fable, and reviewer-terra agents.
• Add matching candidate-* writers for Arena worktrees.
• Route Reflect judgment to Sol xhigh, tooling to Grok xhigh, and divergent review to Fable max.
• Restore model-diverse lanes in How critique, Interrogate, Peer Review, Arena, and Architect.
• Keep model selectors, thinking levels, contexts, and empty fallback lists in pi/settings.json.
• Add check-model-diversity.mjs to reject missing roles, duplicate reviewer selectors, mutation tools on reviewers, disabled selectors, and skill routing drift before installation.

The audit covered PRs #58, #60, #61, and #62. PR #58 introduced the collapsed lanes. PRs #60 and #61 retained explicit Fable and peer-review routing. PR #62 changes implementation roles but does not introduce another model-diversity loss.

Tradeoffs

The dedicated roles add eight small agent profiles. This keeps model selection out of workflow calls and gives status output a stable role name.

The direct xAI Grok selector currently returns a credit error. The Grok roles use the verified opencode/grok-4.6 selector instead.

Blast Radius

The change affects global Pi agent and settings links after install.sh skills runs and Pi restarts. The current live Pi process remains unchanged. PR #62 touches some of the same ppstack files and may need a rebase after one PR merges.

Verification

• node skills/scripts/check-model-diversity.mjs passed.
• bash -n install.sh skills/scripts/link-skills.sh passed.
• jq empty pi/settings.json passed.
• Frontmatter and relative-link validation passed for all changed Markdown files.
• A temporary-HOME install.sh skills run linked every new role and the tracked settings.
• No-tools probes returned the expected response from Sol xhigh, OpenCode Grok xhigh, Fable max, and Terra xhigh.
• Comment Sicko found no comments to delete and no structural findings.
• git diff --check passed.