ci: make OpenCode host e2e informational (continue-on-error)

OpenCode 1.15.x has a Linux-only HTTP server bring-up bug — the server
binds the port and prints "Database migration complete" but then never
serves /doc, so our test harness's waitForReady() times out at 5 min
per test FILE. macOS local runs with the same 1.15.x binary execute
the same suite in seconds. Pinning to 1.15.4 was supposed to dodge it
(2899eb7) but the symptom returned on the next CI run, meaning the
regression is older or 1.15.4 has the same bug under different
conditions on the GHA Linux runner.

Until upstream OpenCode ships a verified-good Linux build we can pin
to, this job runs informationally but doesn't gate CI or releases.
Coverage is preserved:
- Pi host e2e also spawns the OpenCode harness for the cross-harness
  test (which runs locally fine; CI-only flake is now skipped via the
  it.skipIf in 87b4d92).
- Docker e2e exercises the install/smoke path independently.
- Plugin unit suite covers cache-stability, transform behavior,
  historian publish, tag-owner identity, synthetic todowrite, etc.

The infinite-loop production bug this suite originally caught (95%
emergency notification re-firing, fixed in 48d0040) is locked in by
transform-compartment-phase.test.ts.