fix(#88): wire Xenon complexity gate into CI

[longieirl]

Summary

Wires a blocking Xenon complexity gate into CI. radon was already installed in requirements/ci.txt but no CI step enforced any threshold — this closes that gap.

Three functions in the codebase currently exceed grade C. Rather than using || true (advisory only), the gate is blocking from day one with explicit, documented per-file exclusions for the pre-existing outliers.

Changes

• .github/workflows/ci.yml: add Complexity gate (Xenon) step to the security job
  • Thresholds: --max-average A --max-modules B --max-absolute C
  • Three files excluded via --exclude with inline justification comments (see below)
  • No || true — gate is hard from day one
• requirements/ci.txt: replace radon==6.0.1 + mccabe==0.7.0 with xenon>=0.9.0
  • Xenon depends on Radon; mccabe was installed but unused

Exclusions and rationale

File	Function	Grade	Justification
analysis/table_detector.py	_detect_text_based_table	E (32)	Temporary — pdfplumber word-coordinate coupling blocks safe decomposition without characterisation tests. Tracked: #98
templates/template_detector.py	get_detection_explanation	D (22)	Stable — diagnostic method; exhaustive path coverage of all scoring outcomes is inherent to its purpose
commands/init.py	init_directories	D (26)	Stable — CLI entrypoint orchestration; complexity is setup sequencing, not extractable decision logic

Type

• Bug fix
• New feature
• Breaking change
• Refactoring
• Documentation
• Security

Testing

• Tests pass (coverage ≥ 91%)
• Manually tested
• make docker-integration passed locally (required when touching Dockerfile, entrypoint.sh, docker-compose.yml, or packages/parser-core/)

Local gate verification: xenon --max-average A --max-modules B --max-absolute C --exclude "..." packages/parser-core/src → exit 0

Checklist

• Code follows project style
• Self-reviewed
• Documentation updated (if needed)
• No new warnings

Downstream impact

• This PR changes a public interface in bankstatements_core (exported class, function, or exception)