docs(portfolio): add evidence-backed conversion assets

[mrnicholasbcarter-code]

Summary

• add a recruiter/client-facing portfolio proof matrix tied to repository evidence
• add a scoped AI Gateway Assurance Audit offer with inputs, deliverables, timeline, exclusions, privacy boundary, and safe contact path
• link the assets from README and the public evidence index
• preserve the claims ledger boundary: no unsupported performance, adoption, production-readiness, or model-intelligence claims

Verification

• python scripts/verify_proof_matrix.py
• PYTHONPATH=. pytest -q tests/test_proof_matrix.py tests/test_flagship_demo.py tests/test_documentation_preflight.py (37 passed)
• pre-commit ruff, format, and strict mypy checks

Closes #124 (bounded portfolio conversion-assets slice).

[github-actions]

📊 Benchmark Results

coding

Metric	Fixed	Verdict	Delta
Success Rate	98.0%	99.5%	1.5%
Cost/req	$0.014201	$0.006775	52.3%
p95 Latency	142ms	144ms	2ms

reasoning

Metric	Fixed	Verdict	Delta
Success Rate	100.0%	99.5%	-0.5%
Cost/req	$0.006667	$0.004471	32.9%
p95 Latency	148ms	142ms	-6ms

chat

Metric	Fixed	Verdict	Delta
Success Rate	98.0%	98.5%	0.5%
Cost/req	$0.000105	$0.001352	-1187.6%
p95 Latency	73ms	142ms	69ms