feat: enable FakeHarnessAdapter with CODEXHOST_FAKE_HARNESS

[riba2534]

What

Adds an opt-in debug tier: when CODEXHOST_FAKE_HARNESS=1, every external Harness Adapter is replaced with the existing FakeHarnessAdapter.

Why

FakeHarnessAdapter (packages/harness-adapter/src/testing.ts) is a well-built test double with fault injection and call counting, but today it is only reachable from vitest. Debugging renderer injection or Desktop interaction therefore burns real model quota on every iteration.

This makes a "real Host + real Codex Desktop + fake Harness" tier available without touching any production code path.

How

• createExternalHarnessAdapters returns a map of FakeHarnessAdapter instances when the env var is set to 1, and is otherwise untouched
• A stderr notice is emitted on activation so the tier is never mistaken for real output
• FAKE_HARNESS_ENV is re-exported alongside the existing CODEXHOST_*_COMMAND constants

@codexhost/harness-adapter/testing is already a public export in that package's exports map, so this introduces no new surface and does not trip tools/check-boundaries.mjs.

Scope notes

• Unset, empty, or any other value keeps the production Adapter classes — zero behaviour change by default
• FakeHarnessAdapter itself is unmodified, and its export map is unchanged
• Launcher env forwarding was deliberately left out to keep this TypeScript-only; it can follow separately if the packaged macOS/Windows path needs it

Verification

Rebased onto current main (df334f4) and re-run there:

Check	Result
npm run lint	pass
npm run typecheck	pass
packages/host-runtime/	275 passed, 2 skipped (28 files)

npm run test:rust was not run — this change does not touch the Rust crates.

Tests cover both directions: unset returns the real Adapter classes, =1 returns FakeHarnessAdapter while preserving each harnessId.

[chatgpt-codex-connector]

To use Codex here, create an environment for this repo.