docs: advertise the llms.txt documentation index in AGENTS.md - #2

Keep the template in sync with api-platform/skillset#2 — point agents at
https://api-platform.com/docs/llms.txt and llms-full.txt.

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* feat(installer): scaffold AGENTS.md and CLAUDE.md into new projects

New projects ship agent-instruction files by default so AI coding agents
read current API Platform docs instead of stale training data. AGENTS.md
is the cross-agent convention (Cursor, GitHub Copilot, OpenAI Codex,
Gemini); CLAUDE.md is a thin @AGENTS.md import. Opt out with --no-with-agents.

Adds an AgentsScaffold writer (templates/AGENTS.md, templates/CLAUDE.md), a
withAgents option on ScaffoldOptions (default true), and wires both the
Symfony and Laravel scaffolds. Content matches the api-platform/skillset
AGENTS.md. Mirrors create-next-app's agent-file generation.

* docs(installer): advertise llms.txt index in the scaffolded AGENTS.md

Keep the template in sync with api-platform/skillset#2 — point agents at
https://api-platform.com/docs/llms.txt and llms-full.txt.

* refactor(installer): fetch AGENTS.md from skillset instead of bundling it

Make api-platform/skillset the single source of truth: AgentsScaffold now
downloads AGENTS.md from the skillset main branch at scaffold time rather
than shipping a bundled copy that drifts (the template and its later
llms.txt update both had to be hand-synced). The fetch is injectable so
tests stay offline, and a failed download warns and skips rather than
aborting the scaffold. CLAUDE.md (@AGENTS.md) is written inline; the
bundled templates are removed.