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.

src/github/validation/trigger.ts

@@ -13,12 +13,12 @@ import type { ParsedGitHubContext } from "../context";
 
 export function checkContainsTrigger(context: ParsedGitHubContext): boolean {
   const {
-    inputs: { assigneeTrigger, labelTrigger, triggerPhrase, directPrompt },
+    inputs: { assigneeTrigger, labelTrigger, triggerPhrase, prompt },
   } = context;
 
-  // If direct prompt is provided, always trigger
-  if (directPrompt) {
-    console.log(`Direct prompt provided, triggering action`);
+  // If prompt is provided, always trigger
+  if (prompt) {
+    console.log(`Prompt provided, triggering action`);
     return true;
   }