Add coverage floor threshold (--cov-fail-under=75)

[mvillmow]

Summary

• Configure a 75% code coverage floor in pyproject.toml to prevent silent coverage regressions
• Mirror explicit --cov-fail-under=75 flag in both ci.yml and _required.yml for visibility
• Add development guideline to CLAUDE.md documenting the floor and how to reproduce it locally

Implementation Details

The implementation follows the approved plan from issue #148:

1. pyproject.toml: Added [tool.coverage.run] source = ["telemachy"] and [tool.coverage.report] fail_under = 75
2. ci.yml: Added --cov-fail-under=75 to the pytest command (line 34)
3. _required.yml: Added full coverage flags to the unit-tests job (line 185) to ensure the required gate enforces the floor
4. CLAUDE.md: Documented the floor and reproduction steps in Development Guidelines

Verification

Verified all acceptance criteria from the plan:

• ✓ Explicit flag check: pixi run pytest ... --cov-fail-under=75 exits 0 with "Required test coverage of 75% reached"
• ✓ Config-sourced check: pixi run pytest ... --cov=telemachy (without explicit flag) also enforces the floor via pyproject.toml
• ✓ Regression simulation: Removing test file drops coverage to 52%, floor correctly rejects with "FAIL Required test coverage of 75.0% not reached"
• ✓ YAML validation: Both workflow files pass syntax checks
• ✓ TOML parsing: pyproject.toml parses correctly with fail_under = 75
• ✓ Linting: ruff check and yamllint pass

Closes #148

[mvillmow]

LGTM — cov-fail-under=75 configured in pyproject.toml + mirrored in ci.yml and the required _required.yml gate; source pin prevents flag-less false-pass. Correct, complete, well-scoped.