README: add Atlas Cloud sponsor placement

[elementalsouls]

Add sponsor credit in two spots: a "SPONSORED BY" strip under the byline and a dedicated ## Sponsors section above About. Logo uses a element to swap black/white wordmark with GitHub light/dark theme. SVGs cropped to art bounds and stored under assets/sponsors/.

Pull Request

Description

Brief summary of what this PR changes.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New section / capability (non-breaking change which adds functionality)
• Wordlist / catalog expansion (non-breaking)
• Vendor fingerprint / secret pattern / dork addition (non-breaking)
• Documentation update
• Renumbering / restructuring (potentially breaking)
• Test addition (smoke-test prompt)

Affected skills

• osint-methodology
• offensive-osint
• Both
• Repo infrastructure only (READMEs, docs, CI, etc.)

Affected sections (if applicable)

• methodology §____
• arsenal §____

Checklist

• My change is OSINT-only (no active exploitation, post-exploit, malware tradecraft).
• I updated CHANGELOG.md under [Unreleased].
• I updated the README's "What's in the box" tables (if I added a section).
• I added trigger phrases to the YAML frontmatter (if I added a new triggerable concept).
• I added a self-test prompt to tests/smoke-test-prompts.md (if I added a new capability).
• Severity / detectability / confidence tags are consistent with existing rubrics.
• I tested locally (installed the modified skill in Claude Code and verified the relevant prompts).
• My commits follow the format <type>(<scope>): <subject>.

Sample prompt that exercises this change

[paste a prompt here that triggers the new content]

Expected response

[describe what Claude should now do for that prompt]

Related issues

Closes #___
References #___

Additional notes

(Any breaking changes, migration notes, or context for reviewers.)