fix(eve): merge system instructions before model calls

[iroiro147]

Summary

• merge assembled system instructions into a single system message before constructing the model call
• keep session history, dynamic instruction lifecycle, and array-form user content unchanged
• preserve Anthropic direct-cache provider options by applying the cache marker before merging

Fixes #596.

Tests

• pnpm --filter eve exec vitest run --config vitest.unit.config.ts src/harness/tool-loop.test.ts (148 passed)
• pnpm --filter eve run typecheck
• pnpm --filter eve exec oxlint src/harness/tool-loop.ts src/harness/tool-loop.test.ts src/harness/instrumentation-runtime-context.ts src/public/instrumentation/index.ts
• pnpm exec oxfmt --check packages/eve/src/harness/tool-loop.ts packages/eve/src/harness/tool-loop.test.ts packages/eve/src/harness/instrumentation-runtime-context.ts packages/eve/src/public/instrumentation/index.ts .changeset/merge-system-instructions.md
• pnpm guard:invariants
• pnpm changeset status --since origin/main
• git diff --check

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