Document Pi model, credential, and instruction-file caveats

Under HARNESS=pi the channel owner supplies only the GitHub token; model
access comes from HARNESS_AUTH. Name the variables each Pi mode reads,
state that Pi silently substitutes its default model for an unknown
MODEL (including the default gpt-6-luna), and record that Pi 1.0.128
loads AGENTS.md from the sandbox working directory and its parents on
the host with no option to disable it. Scope the remaining Copilot
entitlement wording in README, hosted-agent, and self-hosting to the
default copilot-sdk harness.

Part of #181 (slice 5 of 5).

Assistant-model: Claude Opus 5.5
Assistant-workflow: harness-stack (run 5f1c7488-e4d3-40e8-8307-591dbcff5d63)
Assistant-duration: unmeasured converged, estimated 20m
Assistant-verification: Bun format passed: mise exec -- bun run fix (existing unrelated oxlint warning)
Assistant-verification: Bun ci passed: mise exec -- bun run ci (existing unrelated oxlint warning)
Assistant-verification: Bun typecheck passed: mise exec -- bun run types (all workspaces and E2E)
Assistant-verification: Bun unit passed: mise exec -- bun test (1591 pass, 2 PostgreSQL skips)
Assistant-verification: Playwright E2E failed: exact locked bun run e2e (229 pass, 2 AGENT=off responsive-workspace UI timing failures; harness case passed)
Assistant-verification: Playwright CI-profile E2E passed: CI=1 locked bun run e2e (228 pass, 3 flaky recovered on retry, exit 0)
Co-authored-by: Alex Lavaee <lavaman131@github.com>