Exercise the Planner harness path with a fake MCP server

Add an isolated AGENT=on browser project with a test-only HarnessV1 and
Streamable HTTP GitHub MCP server. Script read_plan and repository-bound
list_pull_requests through the real host tools, conversation projection,
socket stream and durable transcript while preserving the AGENT=off suite.

Pin Pi 1.0.128 and retain an opt-in real-adapter contract reproduction.
Structured output throws HarnessCapabilityUnsupportedError upstream, so
Pi is not admitted to the production harness map. No Pi Edge result is
claimed until that prerequisite passes and its auth mode is reviewed.

Part of #181 (slice 5 of 5).

Assistant-model: GPT-6 Sol
Assistant-workflow: harness-stack (run 5f1c7488-e4d3-40e8-8307-591dbcff5d63)
Assistant-duration: 28m blocked, estimated 70m
Assistant-verification: Bun Pi adapter contract failed: real createPi Output.object throws HarnessCapabilityUnsupportedError in pinned 1.0.128
Assistant-verification: Bun focused unit passed: harness contract/map tests 9/9; Pi opt-in not enabled
Assistant-verification: Bun unit passed: mise exec -- bun test (1581 pass, 2 PostgreSQL skips)
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: Playwright harness E2E passed: locked bun run e2e --project=harness (1/1 scripted real tool/socket/reload turn)
Assistant-verification: Playwright E2E failed: exact non-CI locked bun run e2e (228 pass, 3 unchanged UI timing failures)
Assistant-verification: Playwright CI-profile E2E passed: CI=1 locked bun run e2e (229 pass, 2 flaky recovered on retry)
Assistant-verification: qlty metrics and smells passed: changed contract and E2E fake analyzed, informational branch/return findings
Assistant-verification: git diff --check passed: staged changes have no whitespace errors
User-preference: Run live Edge checks with github-copilot/gpt-6-luna without changing other tabs or sign-in settings.
Co-authored-by: Alex Lavaee <lavaman131@github.com>