fix(auth-utils): improve error message clarity for destroyed state

Addresses Copilot AI comment on PR #79 about misleading error message.

Problem:
Error message said "Transaction capability destroyed - socket closed" but
destroyed flag can be set in contexts other than socket closure, making
the message potentially misleading or confusing.

Examples when destroyed=true but socket may NOT be closed:
1. Manual cleanup during testing
2. Premature destroy() call due to bug
3. Destroy called but socket still open (edge case)

Old message:
"Transaction capability destroyed - socket closed"
- Assumes socket is closed
- Misleading if socket still open
- Implies correlation that may not exist

New message:
"Transaction capability destroyed - cannot initiate new transactions"
- States exact condition (destroyed=true)
- Explains direct consequence (no new transactions)
- No assumption about socket state
- More accurate and helpful for debugging

Impact:
- Improved error clarity for developers debugging issues
- No assumption about WHY destroyed is true
- Focus on WHAT the error means (can't start transaction)
- Better for logs and error tracking

This is a low-severity improvement (message clarity only), but addresses
valid feedback about potentially confusing error messaging.

https://claude.ai/code/session_VMxqX

src/Utils/auth-utils.ts

@@ -320,7 +320,7 @@ export const addTransactionCapability = (
 					// CRITICAL: Check destroyed flag INSIDE mutex to prevent race condition
 					// This ensures atomic check-and-execute: if we acquire mutex, resources exist
 					if (destroyed) {
-						throw new Error('Transaction capability destroyed - socket closed')
+						throw new Error('Transaction capability destroyed - cannot initiate new transactions')
 					}
 
 					const ctx: TransactionContext = {