Enforce 95% Python SDK coverage in CI

[negillett]

Summary

• Gate IntentProof CI: Build and Test at 95% line coverage on
  src/intentproof/ via pytest-cov and scripts/check-coverage.sh.
• Add end-to-end tests for the local loop (configure, wrap, outbox,
  HTTP export) plus targeted coverage for outbox transactions, async
  wrap, keypair edge cases, and signing verification.

Test plan

• pip install -e ".[dev]" && pytest -q (98.8% line coverage)
• bash ./scripts/check-coverage.sh 95
• CI IntentProof CI: Build and Test green on this PR

[cursor]

PR Summary

Low Risk
Low risk: changes are limited to CI/test tooling and add test coverage; main impact is stricter CI gating that may fail builds if coverage drops.

Overview
CI now enforces 95% line coverage by adding pytest-cov defaults in pyproject.toml, ignoring coverage artifacts in .gitignore, and updating the GitHub Actions workflow to run scripts/check-coverage.sh after tests.

Test coverage is significantly expanded with new end-to-end local loop tests (configure → wrap → outbox → HTTP export) plus targeted edge/branch tests for async wrapping, canonicalization number/type handling, outbox transaction/chain-state behavior, exporter HTTP response handling, keypair load/write race cases, and signature verification failure modes.

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[cursor]

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