feat: implement Claude Code GitHub Action v1.0 with auto-detection and slash commands

Major features:
- Mode auto-detection based on GitHub event type
- Unified prompt field replacing override_prompt and direct_prompt
- Slash command system with pre-built commands
- Full backward compatibility with v0.x

Key changes:
- Add mode detector for automatic mode selection
- Implement slash command loader with YAML frontmatter support
- Update action.yml with new prompt input
- Create pre-built slash commands for common tasks
- Update all tests for v1.0 compatibility

Breaking changes (with compatibility):
- Mode input now optional (auto-detected)
- override_prompt deprecated (use prompt)
- direct_prompt deprecated (use prompt)

src/modes/detector.ts

@@ -0,0 +1,77 @@
+import type { GitHubContext } from "../github/context";
+import {
+  isEntityContext,
+  isAutomationContext,
+  isPullRequestEvent,
+  isIssueCommentEvent,
+  isPullRequestReviewCommentEvent,
+} from "../github/context";
+import { checkContainsTrigger } from "../github/validation/trigger";
+
+export type AutoDetectedMode = "review" | "tag" | "agent";
+
+export function detectMode(context: GitHubContext): AutoDetectedMode {
+  if (isPullRequestEvent(context)) {
+    const allowedActions = ["opened", "synchronize", "reopened"];
+    const action = context.payload.action;
+    if (allowedActions.includes(action)) {
+      return "review";
+    }
+  }
+
+  if (isEntityContext(context)) {
+    if (
+      isIssueCommentEvent(context) ||
+      isPullRequestReviewCommentEvent(context)
+    ) {
+      if (checkContainsTrigger(context)) {
+        return "tag";
+      }
+    }
+
+    if (context.eventName === "issues") {
+      if (checkContainsTrigger(context)) {
+        return "tag";
+      }
+    }
+  }
+
+  if (isAutomationContext(context)) {
+    return "agent";
+  }
+
+  return "agent";
+}
+
+export function getModeDescription(mode: AutoDetectedMode): string {
+  switch (mode) {
+    case "review":
+      return "Automated code review mode for pull requests";
+    case "tag":
+      return "Interactive mode triggered by @claude mentions";
+    case "agent":
+      return "Automation mode for scheduled tasks and workflows";
+    default:
+      return "Unknown mode";
+  }
+}
+
+export function shouldUseTrackingComment(mode: AutoDetectedMode): boolean {
+  return mode === "tag";
+}
+
+export function getDefaultPromptForMode(
+  mode: AutoDetectedMode,
+  context: GitHubContext,
+): string | undefined {
+  switch (mode) {
+    case "review":
+      return "/review";
+    case "tag":
+      return undefined;
+    case "agent":
+      return context.inputs?.directPrompt || context.inputs?.overridePrompt;
+    default:
+      return undefined;
+  }
+}