Fix review findings: constants, deps, and fetch robustness

[firede]

Summary

• replace magic number in markdown sampling with named constant
• run go mod tidy to correct direct/indirect dependency classification
• return explicit error on HTTP 4xx/5xx responses instead of treating error pages as success
• tighten markdown detection to avoid classifying JSON payloads as markdown
• fix browser header conversion for multi-value Cookie headers (; separator)
• add regression tests for the above behavior

Validation

• go test ./...
• go test -race ./...

[chatgpt-codex-connector]

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