feat: add first-class Pi runtime integration

[regenrek]

Summary

• add first-class pi support to project init, install, doctor, prompts, observed-client evidence, and agent routing surfaces
• install native repository-local .pi/skills plus optional pi-subagents worker/reviewer roles, while explicitly keeping MCP, hooks, settings, extensions, credentials, and --client all out of Pi V1
• publish the frozen Pi V1 contract, typed /skill:planr agent prompt recipe, integration guide, generated reference updates, and deterministic inventory/contract coverage

Live validation

• Pi upgraded from 0.81.1 to 0.82.1
• pi-subagents 0.37.0 installed
• core smoke: effective parent openai-codex/gpt-5.6-sol, marker PI_CORE_SMOKE_OK
• orchestration smoke: SOL 5.6 launched one fresh planr-reviewer child with effective xai/grok-4.5, exit 0, no fallback, complete Planr review verdict, and no tracked file mutation
• reviewer policy introspection: acceptanceRole: read-only and completionGuard: false

Raw prompts/completions and credentials are intentionally not stored; Planr retains privacy-safe model/status/marker summaries.

Verification

• cargo fmt --check
• cargo test
• cargo clippy --all-targets -- -D warnings
• docs content, typecheck, lint, agent recipe/journey/landing/Markdown contracts
• CLI/MCP reference generation and checks
• production static docs deployment dry-run and release-readiness checks
• repository privacy, staged-file, BetterLeaks, and Trivy gates
• independent findings-first review: complete, no actionable findings

Residual boundary

The optional reviewer keeps bash for read-only verification commands. pi-subagents treats it as read-only policy via explicit frontmatter, but bash is inherently mutation-capable and is not a sandbox.