docs: September 2026 receipts positioning vs Cedar sidecars

[SafetyMP]

Summary

• Reposition FidusGate as signed Ed25519 receipts for MCP tool calls plus a runnable admin console (ledger, simulator, verifier), not a generic zero-trust agent governance platform.
• Contrast OpenFirma (sidecar enforcement without this console/receipt UX). Document next slice: publish an MCP server listing if the gateway is real; drop the mcp topic if it is only a label.
• Community contract first in AGENTS.md (commands, Cedar/receipt paths, never weaken fail-closed). Site overlay moved to docs/factory-overlay.md. Add llms.txt, .github/copilot-instructions.md, and .github/skills/cedar-mcp-receipts/ (adapted from scopeblind/scopeblind-gateway; no originality claim).

No app code. No Prisma/Vitest majors.

Test plan

• ./scripts/harness/verify.sh — verify: ok (ci/web parity); threat-model ok; cedar CLI not installed locally (skipped)
• Confirm README lede + Why not OpenFirma + demo/screenshots still render
• Confirm GitHub discovers .github/SECURITY.md (## Reporting a Vulnerability)
• Do not merge to main until review; branch is docs/sep-2026-community-revamp only

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