feat: add Azure CLI (az) filter for JSON output and warning suppression

Problem

Azure CLI (az) prepends upgrade warning messages to every command output before the actual JSON result. These warnings ("WARNING: The default value for this argument will change", "Please let us know how you feel about this change") appear before the useful JSON and inflate token usage on every call.

Proposed Solution

Add src/filters/az.toml matching ^az\b that:

• Strips WARNING: prefix lines
• Strips "Please let us know" and "You can use" continuation lines
• Truncates at 120 chars/50 lines for large JSON list output
• Preserves the actual JSON response

Also add a corresponding RtkRule in src/discover/rules.rs with subcmd_savings for common subcommands (aks, vm, network, storage, group, acr).