docs(grid): add accessible intro & how-to guide for Grid

[maximelb]

What

Adds a newcomer-friendly intro/how-to guide for Grid (the AI Field Engineer experience) at docs/9-ai-sessions/grid.md, and links it into the AI Sessions nav.

It's deliberately a mix of marketing framing and practical how-to, written to be accessible to someone who has never touched a D&R rule.

Why

There was no single, plain-language entry point that explains what Grid is, what you get, and how to start. Newcomers land on the marketing /deploy page but had nowhere in the docs to go next.

What it covers

• The FDE concept — a persistent AI Field Deployed Engineer that supervises and builds dedicated worker agents, rather than doing the per-event work itself.
• Built on battle-hardened infrastructure — everything Grid builds is standard LimaCharlie config running on the same platform MSSPs and security teams already use at scale; no AI black box.
• The end-to-end lifecycle — state the outcome → connect data → review charter → build → see the story map → ongoing supervision.
• A first-few-minutes quickstart with concrete example prompts.
• Control & safety — approval gates, least-privilege scoped keys, full audit trail.
• Visibility & rollback — the story map and one-tag removal of an FDE's entire footprint.
• Chat with FDE, cost/billing transparency, and links onward to the deeper AI Sessions docs.

Content is grounded in the Grid onboarding/FDE prompts in web-app-frontend (src/lib/aiFde.ts) and the Deploy marketing surface — no invented claims or numbers.

Notes

• markdownlint-cli2 passes clean (0 errors).
• All internal links verified to resolve to existing pages.
• No tone cheese; outcomes-first, plain language throughout.

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