Run Planner through Copilot HarnessAgent

Move the hosted Planner to a disposable HarnessAgent turn backed by a
host-process Copilot SDK adapter. Keep worker-only Copilot sessions isolated
until their structured-output migration, and fail closed on unexpected tools.

Assistant-model: GPT-6 Sol, Claude Sonnet 5, GPT-6 Luna
Assistant-workflow: harness-stack (run e1e96553-487c-4e7f-acb9-9298606eb64c)
Assistant-duration: 70m converged, estimated 90m
Assistant-verification: Bun adapter contract unit passed: Copilot SDK stub, internal result tool, abort, metadata variants
Assistant-verification: Bun metadata regression passed: no-alias live shape failed before fix and passed after; builtin alias and duplicate substitution rejected
Assistant-verification: Bun ci passed: mise exec -- bun run ci (dprint, oxlint, tokens)
Assistant-verification: Bun types passed: mise exec -- bun run types (all workspaces and E2E)
Assistant-verification: Bun test passed: mise exec -- bun test (1567 pass, 2 PostgreSQL skips)
Assistant-verification: Playwright E2E failed: first parallel run 228 passed, 2 responsive-workspace fixture transition failures
Assistant-verification: Playwright E2E passed: flock /tmp/chopin-e2e.lock mise exec -- bun run e2e --workers=1 (230/230 on final checkout)
Assistant-verification: agent-browser Edge E2E passed: signed-in @chopin cited README heading and PR #189, expanded two host tools, refused foreign repository
Assistant-verification: ffmpeg video passed: 182.967s WebM showed scene changes beyond first second
Assistant-verification: oxlint negative probe passed: rejected type-only Copilot SDK import outside adapter
Assistant-verification: qlty metrics and smells passed: analyzed changed session, agent and chat sources; existing complexity reported
User-preference: Keep slice PRs draft until the whole stack is ready; upload only inspected feature-proof evidence.
User-preference: Run Edge checks on github-copilot/gpt-6-luna in the user's Edge without changing other tabs or sign-in settings.
Co-authored-by: Alex Lavaee <lavaman131@github.com>