Prepare repo for launch: fix new-install state leaks, add SECURITY.md and chat template

[Copilot]

New installations via setup.sh or the template repo inherit the source repo's Spock identity (AGENTS.md) and expensive model config (claude-opus-4-6 / high thinking). Users also have no general-purpose issue template and no SECURITY.md.

Changes

• setup.sh — Reset AGENTS.md and .pi/settings.json to default templates after copying, alongside the existing state/ cleanup
• install/settings.json — New default template: claude-sonnet-4-20250514 / low thinking (matches README examples)
• install/MINIMUM-INTELLIGENCE-INSTALLER.ts — Now also copies the chat issue template and initialises settings.json with defaults
• SECURITY.md — Root-level security policy pointing to existing docs (security assessment, capabilities analysis, incident response)
• 💬 Chat issue template — General-purpose conversation template in both install/ and .github/ISSUE_TEMPLATE/, complementing the existing 🥚 Hatch template

# setup.sh now resets repo-specific state for fresh installs
rm -rf "$TARGET_DIR/state"
cp "$TARGET_DIR/install/MINIMUM-INTELLIGENCE-AGENTS.md" "$TARGET_DIR/AGENTS.md"
cp "$TARGET_DIR/install/settings.json" "$TARGET_DIR/.pi/settings.json"

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