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/install-mcp-server.test.ts

@@ -24,7 +24,6 @@ describe("prepareMcpConfig", () => {
     entityNumber: 123,
     isPR: false,
     inputs: {
-      mode: "tag",
       prompt: "",
       triggerPhrase: "@claude",
       assigneeTrigger: "",
@@ -32,8 +31,6 @@ describe("prepareMcpConfig", () => {
       allowedTools: [],
       disallowedTools: [],
       customInstructions: "",
-      directPrompt: "",
-      overridePrompt: "",
       branchPrefix: "",
       useStickyComment: false,
       additionalPermissions: new Map(),