Run CLI Compose tests on Platform ARM64 runners

[rmorehig]

Part of RAW-540.

Run the CLI Compose integration job on Platform's arc-runner-set-platform-arm64 pool. Install pkg-config and libssl-dev, which are absent from the ARC image and required by the CLI's native TLS dependency. The first live attempt reached a healthy stack but failed compiling openssl-sys; installing these prerequisites fixes that failure.

Preserve the trusted same-repository PR guard and existing image build/reuse behavior. Document the runner pool in the README. Image optimization remains a follow-up under RAW-540.

Validation at f163a6f:

• CI run 37949335924 passed: the standard CLI tests and all six real Platform E2E tests, including the four workflow cases.
• Actual runner: arc-runner-set-platform-arm64-gxc46-runner-n5r9f. Runner access, checkout, Docker login, image build, healthy stack startup, bootstrap, native compilation, tests, and cleanup all succeeded.
• YAML parsing and diff checks passed. Strict maintainability review via autoreview --mode local --max-priority P3 --no-web-search reported no actionable findings.

Observed ARC job timings, excluding queue time (9 October 2026, 15:08:23–15:14:31 UTC):

Stage	Duration
Full Compose job	6m 08s
Frontend image build	3m 16s
Container creation / remaining pulls	36s
Stack startup	34s
CLI compilation and E2E step	48s
Six E2E tests themselves	5.82s

The earlier hosted-runner job took 22m59s, including a 21m15s image build. This is not an isolated runner speedup: the successful ARC run reused the matching published backend (REUSED_BACKEND=true), while the earlier run rebuilt it. Frontend build and reliable reuse of matching Platform images remain the next optimization targets.

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Note

Low Risk
CI and documentation only; no application code, auth, or runtime behavior changes.

Overview
Moves the CLI against Platform Docker Compose integration job from GitHub-hosted ubuntu-24.04-arm to Platform’s arc-runner-set-platform-arm64 runner pool.

Adds a workflow step to install pkg-config and libssl-dev on that image so the CLI can compile its OpenSSL/TLS native dependency (openssl-sys), which failed on the lean ARC environment. Same-repo PR guards and backend image reuse/build logic are unchanged.

README now documents the ARC runner pool instead of a generic ARM64 hosted runner.

^{Reviewed by Cursor Bugbot for commit f163a6f. Bugbot is set up for automated code reviews on this repo. Configure here.}