Rewrite README hero section for 30-second clarity

[dgenio]

What changed

Rewrites the top of README.md as a compact hero section so first-time visitors understand ChainWeaver in under 30 seconds — within a single browser viewport.

Hero section (new, lines 1–42)

• One-line tagline: "Compile deterministic MCP tool chains into LLM-free executable flows."
• Badges: PyPI version, CI status, Python versions, license
• Mermaid diagram: before/after showing N LLM calls → 0 LLM calls
• Minimal quickstart snippet: ≤10 lines of runnable code
• Navigation links: Installation · Why ChainWeaver? · Quick Start · Architecture · Roadmap

Content reorganization

• Merged "The Problem" + "The Solution" into a new "Why ChainWeaver?" section below the fold
• Moved the detailed comparison table into "Why ChainWeaver?" and removed the duplicate at the bottom
• Promoted Installation to a top-level ## section (was nested under Quick Start)

Bug fix (adjacent)

• Fixed Architecture tree: logging.py → log_utils.py (the actual filename)

Why

Resolves #66 — the README was thorough but dense; a first-time visitor had to scroll through multiple sections before understanding what ChainWeaver does.

How verified

ruff check chainweaver/ tests/ examples/     → All checks passed
ruff format --check chainweaver/ tests/ examples/ → 13 files already formatted
python -m mypy chainweaver/                  → no issues found in 7 source files
python -m pytest tests/ -v                   → 46 passed
python examples/simple_linear_flow.py        → runs correctly, output verified

Tradeoffs / risks

• Mermaid diagram uses GitHub-compatible syntax; may not render in all Markdown viewers
• No code changes — only README.md touched

Scope notes

• All existing content is preserved (reorganized, not deleted)
• The logging.py → log_utils.py fix in the Architecture section corrects a factual error
• No changes needed to AGENTS.md or copilot-instructions.md (verified anchor links still resolve)

[Copilot]

Pull request overview

Updates the top-level README to improve first-time visitor “time-to-understanding” by adding a compact hero section, reorganizing introductory content, and correcting an Architecture filename reference.

Changes:

• Replaces the opening README content with a hero section (tagline, badges, Mermaid diagram, minimal quickstart, navigation links).
• Reorganizes “Problem/Solution” content into a single “Why ChainWeaver?” section and removes the duplicated comparison table.
• Fixes the Architecture tree filename reference from logging.py to log_utils.py.

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