ci(opencode): bump pinned OpenCode to v1.18.25

[cortexkit-ci]

Automated bump of .github/opencode-version.txt from 1.18.21 to 1.18.25.

Upstream release notes: https://github.com/anomalyco/opencode/releases/tag/v1.18.25

What this changes

All three E2E harnesses now install OpenCode v1.18.25:

• 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 1.18.21 until this PR is merged.

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

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Summary by cubic

Bumps the pinned OpenCode version from 1.18.21 to 1.18.25, so all three E2E harnesses run the new release. If any platform regresses in CI, hold the bump and file an upstream issue.

^{Written for commit 0db0978. Summary will update on new commits.}

Greptile Summary

The PR updates the centrally managed OpenCode E2E version pin from 1.18.21 to 1.18.25.

• Linux Docker and macOS harnesses install the pinned release through the OpenCode installer.
• Windows installs the corresponding opencode-ai npm version.
• Changes to the pin trigger the cross-platform E2E workflow.

Confidence Score: 5/5

The PR appears safe to merge once its required cross-platform E2E checks pass.

The new single-line version value is compatible with each harness’s pin parsing, and the repository routes pin changes through Linux, macOS, and Windows validation.

Important Files Changed

Filename	Overview
.github/opencode-version.txt	Updates the shared OpenCode version pin to 1.18.25; no concrete defect was identified in the changed value or its repository consumers.

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