feat: add inventory json command

[peg]

Summary

• add rampart inventory for local observability/debugging snapshots
• add --json output with schema rampart.inventory.v1
• include protected agents, policy file load status, audit footprint, and local rampart serve reachability
• document the new command in the CLI reference

Validation

• go test ./... -count=1
• go vet ./...
• make build
• git diff --check