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.

action.yml

@@ -24,19 +24,10 @@ inputs:
     required: false
     default: "claude/"
 
-  # Mode configuration (v1.0: auto-detected, kept for backward compatibility)
-  mode:
-    description: "DEPRECATED in v1.0: Mode is now auto-detected. Review mode for PRs, Tag mode for @claude mentions, Agent mode for automation."
-    required: false
-    default: ""
-
   # Claude Code configuration
   model:
     description: "Model to use (provider-specific format required for Bedrock/Vertex)"
     required: false
-  anthropic_model:
-    description: "DEPRECATED: Use 'model' instead. Model to use (provider-specific format required for Bedrock/Vertex)"
-    required: false
   fallback_model:
     description: "Enable automatic fallback to specified model when primary model is unavailable"
     required: false
@@ -53,15 +44,7 @@ inputs:
     required: false
     default: ""
   prompt:
-    description: "Instructions for Claude. Can be a direct prompt, slash command (e.g. /review), or custom template. Replaces override_prompt and direct_prompt from v0.x"
-    required: false
-    default: ""
-  override_prompt:
-    description: "DEPRECATED: Use 'prompt' instead. Kept for backward compatibility."
-    required: false
-    default: ""
-  direct_prompt:
-    description: "DEPRECATED: Use 'prompt' instead. Kept for backward compatibility."
+    description: "Instructions for Claude. Can be a direct prompt, slash command (e.g. /review), or custom template."
     required: false
     default: ""
   mcp_config: