feat: add interactive prompt system for extensions (select, confirm, input)

Extensions can now show modal prompts to the user via ctx.PromptSelect,
ctx.PromptConfirm, and ctx.PromptInput. Prompts render inline below the
separator (replacing the input area) and use channel-based sync so the
extension blocks until the user responds. Extension slash commands run in
dedicated goroutines to avoid stalling BubbleTea's Cmd scheduler.

examples/extensions/prompt-demo.go

@@ -0,0 +1,113 @@
+//go:build ignore
+
+package main
+
+import (
+	"fmt"
+	"strings"
+
+	"kit/ext"
+)
+
+// Init demonstrates the interactive prompt system. It registers three slash
+// commands that show each prompt type (select, confirm, input), plus a
+// combined workflow command that chains prompts together.
+func Init(api ext.API) {
+
+	// /demo-select — shows a selection list.
+	api.RegisterCommand(ext.CommandDef{
+		Name:        "demo-select",
+		Description: "Demo: pick from a list",
+		Execute: func(args string, ctx ext.Context) (string, error) {
+			result := ctx.PromptSelect(ext.PromptSelectConfig{
+				Message: "Choose your deployment target:",
+				Options: []string{"local", "staging", "production"},
+			})
+			if result.Cancelled {
+				return "Selection cancelled.", nil
+			}
+			return fmt.Sprintf("Selected: %s (index %d)", result.Value, result.Index), nil
+		},
+	})
+
+	// /demo-confirm — shows a yes/no confirmation.
+	api.RegisterCommand(ext.CommandDef{
+		Name:        "demo-confirm",
+		Description: "Demo: yes/no confirmation",
+		Execute: func(args string, ctx ext.Context) (string, error) {
+			result := ctx.PromptConfirm(ext.PromptConfirmConfig{
+				Message:      "Are you sure you want to deploy?",
+				DefaultValue: false,
+			})
+			if result.Cancelled {
+				return "Confirmation cancelled.", nil
+			}
+			if result.Value {
+				return "Confirmed! Deploying...", nil
+			}
+			return "Declined. Deployment aborted.", nil
+		},
+	})
+
+	// /demo-input — shows a text input.
+	api.RegisterCommand(ext.CommandDef{
+		Name:        "demo-input",
+		Description: "Demo: free-form text input",
+		Execute: func(args string, ctx ext.Context) (string, error) {
+			result := ctx.PromptInput(ext.PromptInputConfig{
+				Message:     "Enter the release tag:",
+				Placeholder: "v1.0.0",
+			})
+			if result.Cancelled {
+				return "Input cancelled.", nil
+			}
+			return fmt.Sprintf("Release tag: %s", result.Value), nil
+		},
+	})
+
+	// /demo-workflow — chains multiple prompts into a workflow.
+	api.RegisterCommand(ext.CommandDef{
+		Name:        "demo-workflow",
+		Description: "Demo: chained prompt workflow",
+		Execute: func(args string, ctx ext.Context) (string, error) {
+			// Step 1: select environment
+			env := ctx.PromptSelect(ext.PromptSelectConfig{
+				Message: "Step 1/3: Select environment:",
+				Options: []string{"development", "staging", "production"},
+			})
+			if env.Cancelled {
+				return "Workflow cancelled at step 1.", nil
+			}
+
+			// Step 2: enter version tag
+			tag := ctx.PromptInput(ext.PromptInputConfig{
+				Message:     "Step 2/3: Enter the version tag:",
+				Placeholder: "v1.0.0",
+			})
+			if tag.Cancelled {
+				return "Workflow cancelled at step 2.", nil
+			}
+
+			// Step 3: confirm
+			confirm := ctx.PromptConfirm(ext.PromptConfirmConfig{
+				Message: fmt.Sprintf(
+					"Step 3/3: Deploy %s to %s?",
+					tag.Value, env.Value),
+				DefaultValue: false,
+			})
+			if confirm.Cancelled {
+				return "Workflow cancelled at step 3.", nil
+			}
+			if !confirm.Value {
+				return "Deployment declined.", nil
+			}
+
+			var summary strings.Builder
+			summary.WriteString("Deployment summary:\n")
+			fmt.Fprintf(&summary, "  Environment: %s\n", env.Value)
+			fmt.Fprintf(&summary, "  Version:     %s\n", tag.Value)
+			summary.WriteString("  Status:      initiated")
+			return summary.String(), nil
+		},
+	})
+}