Add detailed Mermaid flow diagram to README

[Copilot]

This PR adds a comprehensive Mermaid flowchart to the README that documents the complete webhook processing flow in the Hooks framework.

Changes Made

• Added a new "Architecture Flow" subsection under "How It Works 🔧"
• Created a detailed Mermaid diagram that shows:
  • Server Bootstrap Process: Configuration loading, plugin initialization, and endpoint setup
  • Request Processing Pipeline: Complete flow from incoming request to response
  • Plugin Integration Points: How auth, handler, lifecycle, and instrument plugins integrate
  • Error Handling Paths: Multiple error scenarios and their responses
  • Decision Points: IP filtering, authentication validation, and error handling
  • Visual Styling: Color-coded nodes for different states (start, success, errors)

Technical Details

The diagram accurately reflects the implementation in lib/hooks/app/api.rb and incorporates information from:

• Server bootstrap process in lib/hooks/core/builder.rb
• Plugin system architecture documented in docs/ directory
• Authentication flow, IP filtering, and lifecycle hooks
• Error handling for both handler errors and system errors

The flowchart provides developers with a clear visual understanding of how webhook requests flow through the system, making it easier to:

• Understand the request processing pipeline
• Debug issues at specific stages
• Plan custom plugin implementations
• Contribute to the framework

Testing

• All unit tests pass (591 examples, 0 failures, 100% coverage)
• All integration tests pass (10 examples, 0 failures)
• All acceptance tests pass (52 examples, 0 failures)
• Linting passes with no offenses detected

Fixes #64.

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[Copilot]

Pull Request Overview

This PR introduces a detailed Mermaid flowchart documenting the Hooks framework’s webhook request lifecycle and links it from the main README.

• Added docs/architecture_flow.md containing a comprehensive Mermaid diagram of server boot, request pipeline, plugin hooks, and error paths
• Updated README.md to reference the new Architecture Flow documentation

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
docs/architecture_flow.md	New Mermaid-based flow diagram showing webhook processing flow
README.md	Added link to Architecture Flow section in docs