docs(decisions): ADR-018 propose central agent asset registry (#246)

Draft ADR proposing a versioned, immutable-per-version asset registry
for MCP servers, Cedar policy modules, and skills. Fixes the contract
(semver grammar, immutability, resolve-at-create-task, descriptor
validation, governance workflow) and defers substrate selection to the
design PR. Ranks AWS Agent Registry as preferred with DynamoDB+S3 as
fallback; surveys mcp-gateway-registry, agentregistry.ai, and Entra
Agent Registry+AGT as considered alternatives.

Status: proposed. Requires the #246 `approved` label before opening a
follow-up implementation PR (per ADR-003).

Includes regenerated Starlight mirror.

Author: bgagent