fix: keep OpenCode enrich out of system prompt

[wbugitlab1]

Summary

• Move OpenCode per-file /agentmemory/enrich injection from experimental.chat.system.transform to chat.message message parts.
• Keep first-turn AGENTMEMORY_INSTRUCTIONS and /context system injection unchanged.
• Add a regression test proving /enrich no longer mutates output.system, is appended to output.parts, and is excluded from prompt observation.
• Update the OpenCode README to document stable system context plus volatile message-stream file enrichment.

Fixes #287.

Verification

• corepack pnpm exec vitest run --exclude test/integration.test.ts test/opencode-prompt-cache.test.ts failed before the fix for the expected reasons.
• corepack pnpm exec vitest run --exclude test/integration.test.ts test/opencode-prompt-cache.test.ts
• corepack pnpm exec vitest run --exclude test/integration.test.ts test/opencode-prompt-cache.test.ts test/opencode-auto-context.test.ts test/integration-plaintext-http.test.ts test/context-injection.test.ts test/hook-source-smoke.test.ts
• corepack pnpm run lint
• corepack pnpm run build
• corepack pnpm test
• semgrep scan --config p/default --error --metrics=off .
• gitleaks protect --staged --redact

Final review lanes: security/privacy, test coverage, and maintainability/integration all returned ACCEPT.