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/trigger-validation.test.ts

@@ -22,19 +22,16 @@ import type {
 import type { ParsedGitHubContext } from "../src/github/context";
 
 describe("checkContainsTrigger", () => {
-  describe("direct prompt trigger", () => {
-    it("should return true when direct prompt is provided", () => {
+  describe("prompt trigger", () => {
+    it("should return true when prompt is provided", () => {
       const context = createMockContext({
         eventName: "issues",
         eventAction: "opened",
         inputs: {
-          mode: "tag",
-          prompt: "",
+          prompt: "Fix the bug in the login form",
           triggerPhrase: "/claude",
           assigneeTrigger: "",
           labelTrigger: "",
-          directPrompt: "Fix the bug in the login form",
-          overridePrompt: "",
           allowedTools: [],
           disallowedTools: [],
           customInstructions: "",
@@ -47,7 +44,7 @@ describe("checkContainsTrigger", () => {
       expect(checkContainsTrigger(context)).toBe(true);
     });
 
-    it("should return false when direct prompt is empty", () => {
+    it("should return false when prompt is empty", () => {
       const context = createMockContext({
         eventName: "issues",
         eventAction: "opened",
@@ -62,13 +59,10 @@ describe("checkContainsTrigger", () => {
           },
         } as IssuesEvent,
         inputs: {
-          mode: "tag",
           prompt: "",
           triggerPhrase: "/claude",
           assigneeTrigger: "",
           labelTrigger: "",
-          directPrompt: "",
-          overridePrompt: "",
           allowedTools: [],
           disallowedTools: [],
           customInstructions: "",
@@ -280,13 +274,10 @@ describe("checkContainsTrigger", () => {
           },
         } as PullRequestEvent,
         inputs: {
-          mode: "tag",
           prompt: "",
           triggerPhrase: "@claude",
           assigneeTrigger: "",
           labelTrigger: "",
-          directPrompt: "",
-          overridePrompt: "",
           allowedTools: [],
           disallowedTools: [],
           customInstructions: "",
@@ -315,13 +306,10 @@ describe("checkContainsTrigger", () => {
           },
         } as PullRequestEvent,
         inputs: {
-          mode: "tag",
           prompt: "",
           triggerPhrase: "@claude",
           assigneeTrigger: "",
           labelTrigger: "",
-          directPrompt: "",
-          overridePrompt: "",
           allowedTools: [],
           disallowedTools: [],
           customInstructions: "",
@@ -350,13 +338,10 @@ describe("checkContainsTrigger", () => {
           },
         } as PullRequestEvent,
         inputs: {
-          mode: "tag",
           prompt: "",
           triggerPhrase: "@claude",
           assigneeTrigger: "",
           labelTrigger: "",
-          directPrompt: "",
-          overridePrompt: "",
           allowedTools: [],
           disallowedTools: [],
           customInstructions: "",