feat: simplify to two modes (tag and agent) for v1.0

BREAKING CHANGES:
- Remove review mode entirely - now handled via slash commands in agent mode
- Remove all deprecated backward compatibility fields (mode, anthropic_model, override_prompt, direct_prompt)
- Simplify mode detection: prompt overrides everything, then @claude mentions trigger tag mode, default is agent mode
- Remove slash command resolution from GitHub Action - Claude Code handles natively
- Remove variable substitution - prompts passed through as-is

Architecture changes:
- Only two modes now: tag (for @claude mentions) and agent (everything else)
- Agent mode is the default for all events including PRs
- Users configure behavior via prompts/slash commands (e.g. /review)
- GitHub Action is now a thin wrapper that passes prompts to Claude Code
- Mode names changed: 'experimental-review' → removed entirely

This aligns with the philosophy that the GitHub Action should do minimal work and delegate to Claude Code for all intelligent behavior.

test/prepare-context.test.ts

@@ -220,13 +220,13 @@ describe("parseEnvVarsWithContext", () => {
       ).toThrow("BASE_BRANCH is required for issues event");
     });
 
-    test("should allow issue assigned event with direct_prompt and no assigneeTrigger", () => {
+    test("should allow issue assigned event with prompt and no assigneeTrigger", () => {
       const contextWithDirectPrompt = createMockContext({
         ...mockIssueAssignedContext,
         inputs: {
           ...mockIssueAssignedContext.inputs,
           assigneeTrigger: "", // No assignee trigger
-          directPrompt: "Please assess this issue", // But direct prompt is provided
+          prompt: "Please assess this issue", // But prompt is provided
         },
       });
 
@@ -239,7 +239,7 @@ describe("parseEnvVarsWithContext", () => {
 
       expect(result.eventData.eventName).toBe("issues");
       expect(result.eventData.isPR).toBe(false);
-      expect(result.directPrompt).toBe("Please assess this issue");
+      expect(result.prompt).toBe("Please assess this issue");
       if (
         result.eventData.eventName === "issues" &&
         result.eventData.eventAction === "assigned"
@@ -249,13 +249,13 @@ describe("parseEnvVarsWithContext", () => {
       }
     });
 
-    test("should throw error when neither assigneeTrigger nor directPrompt provided for issue assigned event", () => {
+    test("should throw error when neither assigneeTrigger nor prompt provided for issue assigned event", () => {
       const contextWithoutTriggers = createMockContext({
         ...mockIssueAssignedContext,
         inputs: {
           ...mockIssueAssignedContext.inputs,
           assigneeTrigger: "", // No assignee trigger
-          directPrompt: "", // No direct prompt
+          prompt: "", // No prompt
         },
       });