ci: fall back to GitHub-hosted runners in forks (#13364)

* ci: run core test workflows on GitHub-hosted runners in forks

Blacksmith runners are scoped to the Kilo-Org organisation, so every job
pinned to a blacksmith-* label queues indefinitely in a fork and is
eventually cancelled without running. Core test CI is therefore dead in
forks: contributors cannot validate a change before opening a PR.

Select the runner from github.repository instead of hardcoding it, so the
canonical repo keeps its Blacksmith runners byte-for-byte and forks fall
back to the GitHub-hosted equivalent. The test.yml unit matrix bakes hosts
into JSON, so it routes through emit_settings, which substitutes the two
labels from LINUX_HOST/WINDOWS_HOST.

pull_request runs execute in the base repo, so PRs sent to Kilo-Org are
unaffected; only pushes and PRs inside a fork change behaviour.

Follow-up to (11210), where these guards were trimmed on the review note
"core test infrastructure (if this fails on forks we should look into that
instead)".

Agent-Signature: claude-opus-5-high on behalf of matt wilkie

* ci: drop test.yml from the fork runner fallback

The Linux sandbox suite cannot pass on a GitHub-hosted runner. Ubuntu 24.04
ships kernel.apparmor_restrict_unprivileged_userns=1, which blocks the
Bubblewrap user-namespace bootstrap that backendSupport() probes, and
packages/core/test/kilocode/linux-sandbox.test.ts asserts support.available is
true unconditionally on Linux. Routing test.yml to ubuntu-latest in forks would
therefore turn a cancelled required check into a failing one.

Revert test.yml to its base state and keep the fallback on the five workflows
that do not exercise the sandbox. Making test.yml fork-capable needs a decision
about relaxing that sysctl on CI runners, which is Kilo-Org's call, not
something to smuggle into a runner-selection change.

Reported by the Codex review bot on PR 13364.

Agent-Signature: claude-opus-5-high on behalf of matt wilkie

* ci: re-trigger pipeline (flaky httpapi-pty test on loaded shard)

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Co-authored-by: maphew <486200+maphew@users.noreply.github.com>