docs: add Firecrawl integration design spec

docs/superpowers/specs/AGENTS.md

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+# AGENTS.md
+
+## Scope
+Applies to design specs in this directory.
+
+## Conventions
+- One design spec per file.
+- File naming: `YYYY-MM-DD-<topic>-design.md`.
+- Specs should be implementation-ready: goals, non-goals, design, validation/error handling, and testing.
+- Resolve ambiguity in the document instead of leaving placeholders.
+- If a newer spec supersedes an older one, state that clearly in the newer file.