feat: add JSON output format

[robertsinke]

Summary

Adds a machine-readable JSON output mode via the global --format flag while preserving existing text output as the default.

Supported commands:

• blogwatcher-cli add ... --format json
• blogwatcher-cli remove ... --yes --format json
• blogwatcher-cli blogs --format json
• blogwatcher-cli scan --format json
• blogwatcher-cli articles --format json
• blogwatcher-cli read <id> --format json
• blogwatcher-cli unread <id> --format json
• blogwatcher-cli read-all --yes --format json
• blogwatcher-cli import subscriptions.opml --format json

This makes the CLI easier to use from scripts and agent/tool integrations without parsing human-oriented terminal output.

Notes

• Default behavior remains unchanged: --format text is the default.
• BLOGWATCHER_FORMAT=json works via the existing viper env binding.
• Invalid formats fail early with a clear error.
• Commands that prompt for confirmation in text mode (remove, read-all) require --yes in JSON mode to avoid interactive/non-JSON prompts.
• Docs and the bundled skill mention JSON output for automation.

Testing

go test ./...

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR adds machine-readable JSON output to the CLI tool. A new persistent --format flag controls output as text (default) or JSON. When JSON mode is enabled, the blogs, scan, and articles commands return structured JSON responses instead of human-readable text. The feature includes validation, configuration binding, and comprehensive end-to-end tests.

Changes

JSON Output Feature

Layer / File(s)	Summary
CLI Format Flag and Configuration internal/cli/root.go	Add --format persistent flag to control output and validate the format value during config initialization.
Output Format Serialization Infrastructure internal/cli/output.go	Define output structs for blogs, articles, and scan results with JSON tags. Implement validation helpers, JSON encoder, and transformation functions to convert internal models into JSON-serializable shapes.
Command Output Integration internal/cli/commands.go	Integrate JSON output into blogs, scan, and articles commands by checking JSON mode and returning structured JSON responses. Suppress human-readable console output when JSON mode is active.
End-to-End Tests e2e/e2e_test.go	Test JSON output structure and field validation for blogs, scan, and articles across flag and environment variable modes. Verify that unsupported format values fail with expected error messages.
User Documentation README.md, skills/SKILL.md	Document JSON output feature with CLI examples, environment variable support via BLOGWATCHER_FORMAT, and guidance for agent/automation usage.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 Hops with joy at JSON's birth,
Structured data proves its worth!
Blogs and scans now speak machine,
The cleanest output ever seen. 🎉

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically summarizes the main change: adding JSON output format support to the CLI.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The PR description covers the main objective (JSON output mode), lists supported commands, explains default behavior, testing approach, and relevant notes. All key information is present and well-structured.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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