fix(opencode): concatenate routing block to last system entry instead of splice(1,0)

[aalvarez-dev]

What / Why / How

fix(opencode): concatenate routing block to last system entry instead of splice(1,0)

Strict OpenAI-compatible providers (NaN builders, and potentially others) reject requests with HTTP 400 when multiple role:system messages are present. The OpenCode adapter's experimental.chat.system.transform hook used output.system.splice(1, 0, routingBlock) to inject the routing block and resume snapshot at index 1 — preserving system[0] for OpenCode's cache-fold.

However, OpenCode serializes system[] entries as separate role:system messages for OpenAI-compatible providers, creating a second system message at index 1 that strict providers reject.

Changed both splice(1,0) calls to concatenate to the last system entry — the same pattern used by engram.ts for memory instructions. This sacrifices the cache-fold optimization for cross-provider compatibility. Anthropic and Moonshot are unaffected (they merge system blocks internally).

Affected platforms

• Claude Code
• Cursor
• VS Code Copilot (GitHub Copilot)
• JetBrains Copilot
• Gemini CLI
• Qwen Code
• OpenCode
• KiloCode
• Codex CLI
• OpenClaw (Pi Agent)
• Pi
• Kiro
• Antigravity
• Zed
• All platforms

Test plan

Updated 32 test assertions in tests/opencode-plugin.test.ts to reflect concatenation behavior:

• system.length no longer increases for each injection (routing/snapshot appended to existing entries)
• Index references updated (system[1]→system[0], system[2]→system[last])
• Strict equality (.toBe) changed to partial match (.toContain) for entries with appended content
• Cache-fold test uses join() instead of slice(1,-1) since content is merged into existing entries

All 57 plugin tests pass (npm test). TypeScript typecheck passes (npm run typecheck).

Checklist

• Tests added/updated (TDD: red → green)
• npm test passes
• npm run typecheck passes
• Docs updated if needed (README, platform-support.md)
• No Windows path regressions (forward slashes only)
• Targets next branch (unless hotfix)

Cross-platform notes

Our CI runs on Ubuntu, macOS, and Windows.

• If touching file paths, verify forward-slash normalization on Windows
• If touching hook paths, verify no backslash separators
• Use path.join() / path.resolve(), never hardcode / separators
• Use event-based stdin reading — readFileSync(0) breaks on Windows
• Use os.tmpdir(), never hardcode /tmp