feat(forge): add dynamic agent system and fix Greptile issues

- Add Critical variant to Severity enum in protocol.rs
- Fix is_warning_or_higher() to include Critical severity
- Fix label() method to include Critical case
- Update forge.toml config schema to match ForgeConfig structure
- Add DynamicAgent for loading agents from TOML configuration
- Add AgentRulesFile parser for full agent rules TOML format
- Add load_agents_from_directory() and create_agent() factory functions
- Re-export new config and agent types from forge module

Author: echobt

.cortex/forge/forge.toml

@@ -7,16 +7,18 @@ max_parallel = 4
 timeout_seconds = 300
 # Stop on first failure
 fail_fast = false
-# Output format: "json", "pretty", "minimal"
-output_format = "pretty"
+# Output format: "json", "text", "markdown"
+output_format = "json"
 
-[agents]
-# Enable/disable specific agents
-security = true
-quality = true
-aggregator = true
+[agents.security]
+enabled = true
+priority = 10
 
-# Agent execution order and dependencies
-[dependencies]
-# aggregator runs after security and quality complete
-aggregator = ["security", "quality"]
+[agents.quality]
+enabled = true
+priority = 10
+
+[agents.aggregator]
+enabled = true
+depends_on = ["security", "quality"]
+priority = 5