feat: add delete_prompt built-in meta-prompt

Adds a third built-in meta-prompt for interactively deleting custom prompts
with confirmation. This completes the prompt management trilogy (save, update,
delete).

Changes:
- Add delete_prompt meta-prompt with prompt_name argument
- Interactive workflow: show prompt → confirm → delete via delete_prompt tool
- Update test to expect 3 built-in prompts (was 2)
- Includes safety notes about built-in prompts and backups

The meta-prompt ensures users see what they're deleting and must explicitly
confirm before the delete_prompt tool is called.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

2 files changed, 39 insertions, 3 deletions

diff --git a/internal/promptstore/default_prompts.go b/internal/promptstore/default_prompts.go
index 09a998a..d985500 100644
--- a/internal/promptstore/default_prompts.go
+++ b/internal/promptstore/default_prompts.go
@@ -145,5 +145,41 @@ Start by fetching and showing me the current prompt, then ask clarifying questio
 			Created: now,
 			Updated: now,
 		},
+		{
+			Name:        "delete_prompt",
+			Title:       "Delete Custom Prompt",
+			Description: "Interactively delete an existing custom prompt with confirmation. Claude will show the current prompt, ask for confirmation, and only delete after explicit approval. Built-in prompts cannot be deleted.",
+			Arguments: []PromptArgument{
+				{
+					Name:        "prompt_name",
+					Description: "Name of the existing prompt to delete",
+					Required:    true,
+				},
+			},
+			Messages: []PromptMessage{
+				{
+					Role: "user",
+					Content: MessageContent{
+						Type: "text",
+						Text: `I want to delete the existing prompt '{{prompt_name}}'.
+
+Please help me by:
+1) First, show me the current prompt so I can confirm it's the right one
+2) Ask me to explicitly confirm the deletion
+3) Only after I explicitly confirm, use the delete_prompt tool to delete it
+
+IMPORTANT NOTES:
+- Built-in prompts (save_prompt, update_prompt, delete_prompt) cannot be deleted
+- Only custom prompts stored in user.jsonl can be deleted
+- This action cannot be undone (though backups are automatically created)
+
+Start by showing me the prompt details.`,
+					},
+				},
+			},
+			Tags:    []string{"meta", "prompt-management", "interactive"},
+			Created: now,
+			Updated: now,
+		},
 	}
 }
diff --git a/internal/promptstore/store_test.go b/internal/promptstore/store_test.go
index 6e74b17..2c95ad7 100644
--- a/internal/promptstore/store_test.go
+++ b/internal/promptstore/store_test.go
@@ -84,9 +84,9 @@ func TestJSONLStore_List(t *testing.T) {
 		t.Fatalf("List() error = %v", err)
 	}
 
-	// Should have all prompts (7 user + 2 built-ins)
-	if len(prompts) != 9 {
-		t.Errorf("List() got %d prompts, want 9 (7 user + 2 built-ins)", len(prompts))
+	// Should have all prompts (7 user + 3 built-ins)
+	if len(prompts) != 10 {
+		t.Errorf("List() got %d prompts, want 10 (7 user + 3 built-ins)", len(prompts))
 	}
 
 	// No cursor for full list