feat: Add direct raw file URL for single-file uploads

[konard]

Summary

When uploading a single log file (not split into chunks), the tool now displays a direct link to the raw file content in addition to the repository/gist URL.

• For gists: Returns the permanent raw_url from the GitHub API (accessible without authentication, even for secret gists)
• For repositories: Returns the download_url from the GitHub API (includes token for private repos that enables unauthenticated access)
• Displays a warning that the token expires in ~10 minutes for private repositories

Example output:

✅ Gist created (🔒 private)
🔗 https://gist.github.com/user/abc123
📄 https://gist.githubusercontent.com/user/abc123/raw/hash/file.log

For private repositories:

✅ Repository created (🔒 private)
🔗 https://github.com/user/log-myfile
📄 https://raw.githubusercontent.com/user/log-myfile/main/myfile.log?token=XXX
⚠️  Note: Raw URL token expires in ~10 minutes for private repositories

Changes

• src/index.js: Modified uploadAsGist() and uploadAsRepo() to fetch and return raw file URLs via GitHub API
• src/cli.js: Updated CLI output to display raw URL with appropriate warnings for private repos
• docs/case-studies/issue-21/: Added case study with research findings on GitHub raw URL mechanisms
• experiments/test-raw-url.js: Added integration test script for raw URL functionality
• .changeset/direct-file-link.md: Added changeset for minor version bump

Test plan

• Linting passes (bun run lint)
• Unit tests pass (bun run test)
• Integration test with real gist upload - raw URL is returned and accessible
• Integration test with real repository upload - raw URL with token is returned
• Verified token expiration warning is displayed for private repositories
• Verified no JSON output is echoed during API calls (uses silent mode)

Fixes #21

🤖 Generated with Claude Code

[konard]

🤖 Solution Draft Log

This log file contains the complete execution trace of the AI solution draft process.

💰 Cost estimation:

• Public pricing estimate: $7.082634 USD
• Calculated by Anthropic: $4.947645 USD
• Difference: $-2.134989 (-30.14%)
  📎 Log file uploaded as Gist (809KB)
  🔗 View complete solution draft log

---

Now working session is ended, feel free to review and add any feedback on the solution draft.