docs: add 'The Seven Spheres of Human-AI Co-Development' to docs/

[amrmelsayed]

Summary

Adds The Seven Spheres of Human-AI Co-Development by Waleed Kadous to the in-repo docs/ tree so the article is discoverable alongside docs/faq.md, docs/tips.md, docs/why.md and survives independently of the cluesmith.com blog.

Closes #1099.

Changes

• docs/seven-spheres.md — the article body, plain markdown, verbatim from the authoritative copy in the issue (no rewording, no editorialising). Attribution block (author Waleed Kadous, date 2026-02-10, source URL https://cluesmith.com/blog/seven-spheres) sits directly under the H1, before the cover image. The acknowledgements paragraph and the AI-collaboration footnote are preserved verbatim at the end. No YAML frontmatter, matching the sibling docs.
• docs/assets/seven-spheres-cover.png — the cover image, fetched once from https://cluesmith.com/images/blog/seven-spheres/cover.png and committed as a binary (valid PNG, 2004×1536 RGBA, ~2.8 MB). The article references it as a relative path ./assets/seven-spheres-cover.png, which resolves correctly when viewed on GitHub from docs/.
• codev/state/air-1099_thread.md — builder thread (cohort situational awareness).

Key decisions

All locked in the issue's plan-gate / baked-decisions section; no autonomous design choices were made:

1. Flat location docs/seven-spheres.md (no docs/blog/ or docs/articles/ subdir for a single article).
2. Image filename docs/assets/seven-spheres-cover.png (kebab-case, scoped suffix).
3. Attribution-first placement directly under the H1.
4. No frontmatter (matches faq.md/tips.md/why.md).
5. No cross-linking from other docs / README (out of scope; separate issue if wanted).

Test plan

No tests added — this is pure markdown + a binary image; there is nothing to assert and no CI/runtime behavior changes. Validation performed:

• npm run build → passes (exit 0).
• Unit tests (npm test, e2e excluded) → pass.
• Image verified as a valid PNG via file; relative image path checked against the docs/ location.

Out of scope

Cross-linking from README/other docs, restructuring docs/ into a docs/articles/ subdir, republishing elsewhere, and image optimisation — all deferred per the issue.