feat: add T-Minus-15 section under Products

[BenGWeeks]

Summary

Adds a T-Minus-15 product section to the homepage (#tminus15, after the Thyme section) and a T-Minus-15 entry in the Products nav dropdown (desktop + mobile), after Thyme.

T-Minus-15 is KnowAll's open-source delivery methodology — "Secrets of an Elite DevOps Team". The section copy is sourced directly from the book repo (T-Minus-15/book):

• README.md — what the book is: specific implementation details (roles, estimates, meetings, work-item metadata) for teams applying Agile (Scrum) and DevOps; written for KnowAll's own team members and customers, and open-sourced under Creative Commons.
• executive-summary.adoc — the 15 Steps to Success, delivering high-value features to the business "15 days (or less) at a time".
• chapters/ (buckle-up.adoc, the-avengers-of-agile.adoc, lets-get-agile.adoc) — the five delivery stages (Prep > Design > Engineer > Test > Operate), squad roles (Prepper, Designer, Engineer, Test Pilot, Planner, Product Owner), short sprints, daily 15-minute stand-ups and retrospectives.
• agents/ — persona configs (Poppy the Planner, Pepper the Prepper, Teddy the Tester, Danny the Designer, Ollie the Operator).

Humans AND AI agents

The section deliberately emphasises (in the intro paragraph and a dedicated bullet) that T-Minus-15 is one shared playbook for human team members and AI agents. This framing comes straight from the book's executive summary: the repository ships AI configuration files in /agents so AI co-pilots "follow this playbook and embody the personas", creating "a shared knowledge base that both human and AI team members can operate from" — exactly how KnowAll's own t-minus-15 agent personas work today.

Implementation notes

• Book cover image rendered from the repo's cover/tminus15-cover.pdf (front panel cropped from the print wraparound, resized to 800px tall, optimized to ~62KB) at public/images/products/tminus15-book.png, displayed with a subtle 3D tilt + spine + lime glow against the dark theme.
• CTAs: Read the book → https://github.com/T-Minus-15/book and T-Minus-15 on GitHub → https://github.com/T-Minus-15.
• The Copilots section (and its two cards) swap gray-900/gray-800 backgrounds to preserve the alternating section rhythm now that T-Minus-15 (gray-900) sits after Thyme (gray-800).

STACKED ON: This branch is stacked on feature/thyme-section (PR #28, itself stacked on PR #27 which introduces the Products dropdown). Merge #27 → #28 → this PR in order; the diff shown against master includes the Zapdesk/Thyme commits until those merge.

Screenshots

T-Minus-15 section	Products dropdown

Verification

• npm run build passes
• npm run format:check passes
• Screenshots taken from the dev server via Playwright

🤖 Generated with Claude Code

Fixes #38

Test plan

• Open the homepage, Products dropdown → T-Minus-15
• Verify the section shows the real book cover, methodology summary, humans-AND-agents bullet, and links to the book repo