fix(138): lowercase attack tree PNG filenames to match convention (#139)

render_mermaid_to_png was writing PNG files as {UPPERCASE_ID}-attack-tree.png
while the .md counterparts introduced in #134 enforce lowercase. Added
.lower() to the finding ID in _render_single to produce lowercase filenames
throughout.

Regenerated attack tree PNGs for both example directories using the fixed
script. The examples/agentic-app/sample-report/ set has 7 Critical/High
attack trees (the other 20 .md files in that directory are Medium/Low
findings, which are filtered out per spec 112). The
examples/mermaid-agentic-app/ set has 12 Critical/High attack trees.

Closes #138

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: davidmatousek