docs(architecture): expand diagrams with data-flow and check pipeline detail

[Alberto-Codes]

Problem

The architecture page has one Mermaid flowchart showing the high-level CLI/LSP paths converging on the shared checks layer. While correct, it omits several important paths that contributors and power users need to understand:

• Configuration loading from pyproject.toml
• File discovery mechanisms (git diff, --staged, --all, positional args, --files)
• The exit code / output pipeline (findings → reporting → exit code 0/1/2)
• How freshness diff mode maps git hunks to AST symbols

Mermaid support is already enabled in mkdocs.yml via pymdownx.superfences — we're just underusing it.

Proposed changes

1. Enhance existing high-level flowchart

Add nodes for:

• pyproject.toml config loading
• File discovery (with mode labels: diff/staged/all/positional)
• Reporting output (text/markdown/JSON)
• Exit code determination

2. Add data-flow diagram

A second Mermaid diagram showing the complete data flow:

pyproject.toml → Config → CLI args merge → Discovery mode selection
  → File list → Per-file: AST parse → Check dispatch
  → Findings collection → Reporting format → Exit code

3. Optional: Add freshness-specific diagram

Show the diff-mode pipeline:

git diff → hunks → line ranges → AST symbol mapping → severity classification → findings

This is the most complex check and the one contributors most often need to understand.

Not in scope

• Diagrams on other pages (check pages, rule pages) — keep it on architecture
• Interactive or animated diagrams
• Diagram freshness tracking automation (future docfresh territory)