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/mockContext.ts

@@ -11,7 +11,6 @@ import type {
 } from "@octokit/webhooks-types";
 
 const defaultInputs = {
-  mode: "tag" as const,
   prompt: "",
   triggerPhrase: "/claude",
   assigneeTrigger: "",
@@ -20,8 +19,6 @@ const defaultInputs = {
   allowedTools: [] as string[],
   disallowedTools: [] as string[],
   customInstructions: "",
-  directPrompt: "",
-  overridePrompt: "",
   useBedrock: false,
   useVertex: false,
   timeoutMinutes: 30,