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/modes/types.ts

@@ -3,7 +3,7 @@ import type { PreparedContext } from "../create-prompt/types";
 import type { FetchDataResult } from "../github/data/fetcher";
 import type { Octokits } from "../github/api/client";
 
-export type ModeName = "tag" | "agent" | "experimental-review" | "review";
+export type ModeName = "tag" | "agent";
 
 export type ModeContext = {
   mode: ModeName;
@@ -25,8 +25,8 @@ export type ModeData = {
  * and tracking comment creation.
  *
  * Current modes include:
- * - 'tag': Traditional implementation triggered by mentions/assignments
- * - 'agent': For automation with no trigger checking
+ * - 'tag': Interactive mode triggered by @claude mentions
+ * - 'agent': General automation mode for all events (default)
  */
 export type Mode = {
   name: ModeName;