Add AGENTS.md for AI agent context

[tx2pnw]

Summary

• Adds AGENTS.md to provide AI coding agents (and humans) with structured project context
• Documents directory structure, key concepts (event system, decision system, gateway integration, sidecar plugin)
• Lists decision hook names and gateway filter naming patterns
• Documents common pitfalls (auth mechanisms, async events, WC session handler differences)
• Includes system map and related systems table with repo/location info
• Notes deployment context: standalone repo vs built copy in woocommerce.com

Part of SHILL-1666

Test plan

• Verify AGENTS.md renders correctly on GitHub
• Confirm documented paths and patterns match current codebase

[github-actions]

Package	Line Rate	Complexity	Health
inc/payment-gateways/index.php	0%	0	❌
inc/payment-gateways/lib/stripe.php	63%	77	➖
inc/payment-gateways/load.php	0%	0	❌
inc/payment-gateways/ppcp-gateway.php	31%	26	❌
inc/payment-gateways/stripe.php	26%	17	❌
inc/payment-gateways/transact.php	100%	0	✔
inc/payment-gateways/woocommerce-payments.php	30%	8	❌
inc/rest-api.php	0%	18	❌
inc/sift-decisions/abuse-decisions.php	2%	36	❌
inc/sift-decisions/sift-decision-rest-api-webhooks.php	0%	10	❌
inc/sift-events/class-sift-event-types.php	100%	4	✔
inc/sift-events/normalizers/sift-order.php	96%	6	✔
inc/sift-events/normalizers/sift-payment-gateway.php	100%	5	✔
inc/sift-events/normalizers/sift-payment-method.php	80%	33	✔
inc/sift-events/normalizers/sift-payment-type.php	83%	3	✔
inc/sift-events/normalizers/sift-property.php	100%	3	✔
inc/sift-events/normalizers/sift-verification-status.php	50%	3	➖
inc/sift-events/sift-events-validator.php	82%	142	✔
inc/sift-events/sift-events.php	56%	161	➖
inc/tracking-js.php	0%	6	❌
inc/wc-admin-settings-tab.php	0%	24	❌
index.php	0%	0	❌
sift-for-woocommerce.php	12%	14	❌
Summary	45% (1033 / 2275)	596	❌