fix(validate): correct Upstream comment format to use 'informed by'

Author: marc0olo

docs/concepts/chain-fusion/bitcoin.md

@@ -93,4 +93,4 @@ As deposits accumulate, the minter manages a growing set of UTXOs. Too many smal
 - [Chain Fusion overview](index.md): integration patterns and supported chains
 - [Chain-key cryptography](../chain-key-cryptography.md): threshold ECDSA and Schnorr signing
 
-<!-- Upstream: learn hub staging: chain-fusion/bitcoin-integration.md, chain-fusion/bitcoin-checker-canister.md, chain-fusion/chain-key-bitcoin.md -->
+<!-- Upstream: informed by Learn Hub articles "Bitcoin Integration", "Bitcoin Checker Canister", "Chain-Key Bitcoin" (migrated, source retired) -->

docs/concepts/chain-fusion/chain-key-tokens.md

@@ -66,4 +66,4 @@ The security of a chain-key token rests on two properties:
 - [Chain-key tokens guide](../../guides/digital-assets/chain-key-tokens.md): how to integrate chain-key tokens into an application
 - [Chain-key cryptography](../chain-key-cryptography.md): the threshold signing that makes chain-key tokens possible
 
-<!-- Upstream: learn hub staging: chain-fusion/chain-key-tokens.md -->
+<!-- Upstream: informed by Learn Hub articles "Chain-Key Tokens" (migrated, source retired) -->

docs/concepts/chain-fusion/dogecoin.md

@@ -21,4 +21,4 @@ ckDOGE is the chain-key token representing Dogecoin on ICP, backed 1:1 by real D
 - [Dogecoin canister documentation](https://dfinity.github.io/dogecoin-canister/)
 - [Chain Fusion overview](index.md): integration patterns and supported chains
 
-<!-- Upstream: learn hub staging: chain-fusion/dogecoin-integration.md -->
+<!-- Upstream: informed by Learn Hub articles "Dogecoin Integration" (migrated, source retired) -->

docs/concepts/chain-fusion/ethereum.md

@@ -61,4 +61,4 @@ For a full description of chain-key token architecture, see [Chain-key tokens](c
 - [Chain-key cryptography](../chain-key-cryptography.md): threshold ECDSA signing
 - [Chain-key tokens](chain-key-tokens.md): ckETH and ckERC20 architecture
 
-<!-- Upstream: learn hub staging: chain-fusion/ethereum-integration.md, chain-fusion/evm-rpc-canister.md -->
+<!-- Upstream: informed by Learn Hub articles "Ethereum Integration", "EVM RPC Canister" (migrated, source retired) -->

docs/concepts/chain-fusion/exchange-rate-canister.md

@@ -53,4 +53,4 @@ If the XRC receives largely inconsistent rates from exchanges, it returns an `Ex
 - [HTTPS outcalls](../https-outcalls.md): how the XRC fetches external data
 - [XRC Candid interface](https://github.com/dfinity/exchange-rate-canister/blob/main/src/xrc/xrc.did)
 
-<!-- Upstream: learn hub staging: chain-fusion/exchange-rate-canister.md -->
+<!-- Upstream: informed by Learn Hub articles "Exchange Rate Canister" (migrated, source retired) -->

docs/concepts/chain-fusion/solana.md

@@ -28,4 +28,4 @@ ckSOL is the chain-key token representing SOL on ICP. Like ckETH, it is backed 1
 - [Ethereum integration](ethereum.md): the EVM RPC canister for comparison
 - [Chain-key cryptography](../chain-key-cryptography.md): Ed25519 threshold Schnorr signing
 
-<!-- Upstream: learn hub staging: chain-fusion/sol-rpc-canister.md -->
+<!-- Upstream: informed by Learn Hub articles "SOL RPC Canister" (migrated, source retired) -->