chore: fix unchecked errors in test setup and assertions

[sonupreetam]

Context

Review council on PR #61 flagged systematic unchecked errors in test setup and assertion code across multiple test files. These are pre-existing issues, not regressions from #61.

Files

• cmd/replicator/init_test.go — os.MkdirAll, os.WriteFile, os.ReadFile errors discarded
• cmd/replicator/serve_test.go — os.MkdirAll, os.WriteFile errors discarded
• internal/gitutil/git_test.go — WorktreeAdd, os.WriteFile, Run errors discarded
• internal/forge/worktree_test.go — WorktreeCreate, os.WriteFile, Run errors discarded
• internal/forge/init_test.go, spawn_test.go, review_test.go, progress_test.go, insights_test.go — DB.Exec, DB.QueryRow().Scan() errors discarded
• internal/mcp/server_test.go — json.Unmarshal errors discarded in assertions
• test/parity/parity_test.go — json.Unmarshal errors discarded in setup

Pattern

Replace os.WriteFile(path, data, 0o644) with:

if err := os.WriteFile(path, data, 0o644); err != nil {
    t.Fatalf("setup WriteFile: %v", err)
}

The fix in internal/agentkit/agentkit_test.go (PR #61) shows the correct pattern.

Additional items

• cmd/replicator/init_test.go:TestRunInit_ForceOverwrites uses a negative-only assertion (checks content is different but not what it should be). Should assert equality with embedded content.
• internal/forge/worktree_test.go has no test for the cleanupAll=true branch of WorktreeCleanup, contributing to CRAP score 65.9.