feat(skills): add X source metadata support

[kriptoburak]

Summary

Adds X/Twitter source evidence support to the generated social media analyzer skill.

Context

• The skill already accepts social media data but did not show how to preserve exporter metadata, post URLs, or post IDs for auditable X/Twitter analysis.
• The sample expected total counted saves as engagement, while the campaign calculator omitted saves at campaign level.

Changes

• Adds X/Twitter and Xquik export guidance to SKILL.md and HOW_TO_USE.md.
• Adds an x benchmark alias matching the existing Twitter benchmark.
• Counts saves in campaign-level total engagement.
• Updates sample_input.json, expected_output.json, and the packaged skill zip.

Testing

• Local /review passed (yamllint, JSON schema, Python syntax, markdown links)
• Local /security-scan passed (gitleaks clean, safety audit clean)
• ci-commit-branch-guard workflow passed
• ci-quality-gate workflow passed
• Manual testing completed

Testing Details:

• Ran Python syntax compilation for the changed analyzer modules.
• Parsed both changed JSON files with Python's JSON parser.
• Ran a focused sample validation script comparing sample_input.json against key expected_output.json fields.
• Ran Git whitespace validation on the staged diff.
• Regenerated and listed generated-skills/social-media-analyzer.zip.

Security

• No secrets, credentials, or API keys committed
• Gitleaks scan clean
• Safety dependency audit clean (if applicable)
• No destructive commands in generated outputs
• Path traversal vulnerabilities checked

Documentation

• README.md updated (if applicable)
• CLAUDE.md updated (if applicable)
• Inline code comments added for complex logic
• CHANGELOG.md updated (if applicable)

Reviewers

• @

Related Issues

Related to #1041

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Type: feat
Scope: generated-skills/social-media-analyzer