ci(opencode): bump pinned OpenCode to v1.18.16

[cortexkit-ci]

Automated bump of .github/opencode-version.txt from 1.18.13 to 1.18.16.

Upstream release notes: https://github.com/anomalyco/opencode/releases/tag/v1.18.16

What this changes

All three E2E harnesses now install OpenCode v1.18.16:

• tests/docker/Dockerfile.linux-x64 (Linux Docker E2E)
• tests/macos-e2e/run.sh (macOS native E2E)
• tests/windows-e2e/run.ps1 (Windows native E2E)

Merge gate

The Tests workflow on this PR exercises the new version against the
full E2E matrix. If any platform regresses, do not merge — file
an upstream issue or hold the bump until upstream fixes the
regression. Pin will stay at 1.18.13 until this PR is merged.

Tests runs automatically because this PR was opened with a PAT.
yes

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Summary by cubic

Bump the pinned OpenCode version from 1.18.13 to 1.18.16 via .github/opencode-version.txt, so all E2E harnesses install the new version on Linux, macOS, and Windows. CI runs the full E2E matrix on this PR; merge only if all jobs pass (release notes: https://github.com/anomalyco/opencode/releases/tag/v1.18.16).

^{Written for commit a4642bd. Summary will update on new commits.}

Greptile Summary

Updates the shared OpenCode version pin from 1.18.13 to 1.18.16.

• Causes the Linux, macOS, and Windows E2E harnesses to install OpenCode 1.18.16.
• Also updates consumers of the shared pin, including the benchmark image.

Confidence Score: 5/5

The PR appears safe to merge once its required cross-platform E2E workflow passes.

The change only advances the shared OpenCode pin, and no repository constraints or concrete incompatibilities with version 1.18.16 were identified.

Important Files Changed

Filename	Overview
.github/opencode-version.txt	Bumps the shared OpenCode version pin to 1.18.16; no concrete incompatibility or actionable defect was identified.

_{Reviews (1): Last reviewed commit: "ci(opencode): bump pinned OpenCode to v1..." | Re-trigger Greptile}