feat(config): add Env field to RuntimeConfig and AgentPresetInfo (#860)

Add support for agent presets to specify environment variables that
get exported when starting sessions. This enables agents to use
environment-based configuration.

Changes:
- Add Env field to RuntimeConfig struct in types.go
- Add Env field to AgentPresetInfo struct in agents.go
- Update RuntimeConfigFromPreset to copy Env from preset
- Update fillRuntimeDefaults to preserve Env field
- Merge agent Env vars in BuildStartupCommand functions
- Add comprehensive tests for Env preservation and copy semantics

This is a prerequisite for the OpenCode agent preset which uses
OPENCODE_PERMISSION='{"*":"allow"}' for auto-approve mode.

Co-authored-by: Avyukth <subhrajit.makur@hotmail.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/config/loader.go

@@ -1085,6 +1085,13 @@ func fillRuntimeDefaults(rc *RuntimeConfig) *RuntimeConfig {
 		Args:          rc.Args,
 		InitialPrompt: rc.InitialPrompt,
 	}
+	// Copy Env map to avoid mutation and preserve agent-specific env vars
+	if len(rc.Env) > 0 {
+		result.Env = make(map[string]string, len(rc.Env))
+		for k, v := range rc.Env {
+			result.Env[k] = v
+		}
+	}
 	if result.Command == "" {
 		result.Command = "claude"
 	}
@@ -1252,6 +1259,10 @@ func BuildStartupCommand(envVars map[string]string, rigPath, prompt string) stri
 	if rc.Session != nil && rc.Session.SessionIDEnv != "" {
 		resolvedEnv["GT_SESSION_ID_ENV"] = rc.Session.SessionIDEnv
 	}
+	// Merge agent-specific env vars (e.g., OPENCODE_PERMISSION for yolo mode)
+	for k, v := range rc.Env {
+		resolvedEnv[k] = v
+	}
 
 	// Build environment export prefix
 	var exports []string
@@ -1362,6 +1373,10 @@ func BuildStartupCommandWithAgentOverride(envVars map[string]string, rigPath, pr
 	if agentOverride != "" {
 		resolvedEnv["GT_AGENT"] = agentOverride
 	}
+	// Merge agent-specific env vars (e.g., OPENCODE_PERMISSION for yolo mode)
+	for k, v := range rc.Env {
+		resolvedEnv[k] = v
+	}
 
 	// Build environment export prefix
 	var exports []string