chore(pi): remove tier system after adapter eval gate passes

[laurigates]

What

Remove the pi tier system once the pi adapter (#2090) passes its eval gate. Gated cutover per ADR-0022 and the tool-migration-cutover rule: do not merge before the gate — the adapter must be observed meeting/beating the tier-installed baseline (correct-skill-invocation rate, lower standing tokens/turn) on a local quant.

Complete-sweep checklist (per incomplete-migration-leftover-tooling, find every wiring):

• pi/tiers.yaml (migrate any still-relevant exclude reasons into compatibility frontmatter first — chore(skills): mark Claude-Code-only skills with compatibility frontmatter #2092)
• scripts/install-pi.sh
• scripts/check-pi-tiers.sh + its .pre-commit-config.yaml entry + the Plugin: PR checks workflow step
• justfile pi group recipes: pi-tiers, install-pi, install-pi-domain (keep serve-pi-model; rewrite setup-pi for the adapter flow)
• docs/pi-export.md — rewrite around the adapter; delete the tier/installer sections and the "Out of scope (deferred)" entry this work fulfilled
• CLAUDE.md "Local-model export" table row for pi
• Grep the repo for remaining tiers.yaml / install-pi references

Why

Overlapping solutions (tier manifest + adapter) are worse than either alone: two sources of curation truth that drift. Once the adapter is operational, the manifest is dead weight — but removing it before the gate would leave pi with no working discovery (tool-migration-cutover: config presence is not operation).

Related

ADR-0022, #2089 (eval harness), #2090 (the gate), #2092 (must land first)