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/types.go

@@ -263,6 +263,11 @@ type RuntimeConfig struct {
 	// Empty array [] means no args (not "use defaults").
 	Args []string `json:"args"`
 
+	// Env are environment variables to set when starting the agent.
+	// These are merged with the standard GT_* variables.
+	// Used for agent-specific configuration like OPENCODE_PERMISSION.
+	Env map[string]string `json:"env,omitempty"`
+
 	// InitialPrompt is an optional first message to send after startup.
 	// For claude, this is passed as the prompt argument.
 	// Empty by default (hooks handle context).