Added AI Agentic support

[undiabler]

Hi folks - CTO/entrepreneur here. I've used this template previously (pre "agentic" tooling becoming mainstream), and it helped me ship fast.

While launching a new project, I wanted to reuse SaaS Starter again, but I found it wasnt quite ready for agentic support out of the box: there wasnt a repo-native, hierarchical navigation map that agents can reliably follow to find the "customize here" zones without wandering.

So I am proposing this PR to add that support in a lightweight, template-friendly way.
Goal: make it obvious where to change branding/SEO, pricing/Stripe IDs, auth providers, dashboard placeholders, DB/RLS, email/search utilities, etc.

What was added:

1. Hierarchical AGENTS.md documentation
   • Root AGENTS.md
   • Nested, folder-specific */AGENTS.md files (e.g. src/routes/(marketing)/, src/routes/(admin)/account/, supabase/, etc.)
2. README section: “AI Agents support” with recomended setup.

Two important notes / review request

1. I am NOT a senior frontend developer. I would really appreciate a reviewer with strong frontend/SvelteKit experience to sanity-check that the navigation guidance feels idiomatic and overall correct.
2. These instructions were carefully reviewed and co-authored. This is NOT blindly generated output. I am using a precisely tailored setup (OpenCode + oh-my-opencode from recommendation section).

Why I think this belongs upstream

Since the project already references Kiln AI in the README, I am hoping the core contributors will support enabling broader AI-assisted / agentic use cases for bootstrapping startups - without changing the templates runtime behavior or adding new dependencies.

Scope / risk

• Docs-only change (no runtime logic changes).
• Adds navigation "maps" and explicitly calls out customization zones (auth/billing/RLS/env var boundaries).

Thanks for considering 🙏