feat: add User-Agent header to all HTTP requests (DIS-41)

[ckorhonen]

Add User-Agent header to all HTTP requests

Summary

Adds a User-Agent: opensea-cli/<version> header to every HTTP request made by OpenSeaClient, enabling server-side traffic segmentation by channel (CLI vs MCP vs other consumers) in Datadog/analytics.

• Reads the version dynamically from package.json at module load time using createRequire (standard ESM pattern to avoid rootDir issues with direct JSON imports)
• Adds the header to both get() and post() methods
• Updates existing test assertions to verify the header is present with a semver-format version

Confidence: 🟢 High — small, well-scoped change. Lint, type-check, and all 151 tests pass locally.

Review & Testing Checklist for Human

• Verify createRequire("../package.json") resolves correctly after npm run build — the relative path ../package.json must resolve from dist/ at runtime, not just src/. Run: npm run build && node -e "import('./dist/index.js').then(m => console.log('ok'))" to confirm no runtime import error. This is the primary risk since tests mock fetch and don't exercise the actual createRequire path.
• Confirm the User-Agent value appears in outgoing requests — e.g. make a verbose request and inspect headers, or add a quick log in a local test

Notes

• Test assertions were changed from exact header matching to expect.objectContaining to accommodate the new header without breaking if header order changes. This is marginally less strict than before.
• Test regex uses /^opensea-cli\/\d+\.\d+\.\d+/ to validate semver format rather than just a leading digit.
• Linear ticket: DIS-41
• Devin Session
• Requested by: @ckorhonen

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[devin-ai-integration]

Original prompt from Chris K

Please work on ticket "[Tracking] Add User-Agent header to opensea-cli" ([DIS-41](https://linear.app/opensea/issue/DIS-41/tracking-add-user-agent-header-to-opensea-cli))

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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