fix: remove Fortran zero-config workflow; rely on shell bridge for auto mode (fixes #1166)

[krystophny]

Summary

• Remove Fortran zero-config workflow and indirection; use a simple shell bridge for auto mode.

Scope

• Deleted: src/zero_config/*
• Updated: src/coverage/workflows/coverage_workflow_core.f90 to call scripts/fpm_coverage_bridge.sh when auto mode is requested (alias: --discover-and-gcov/--gcov).
• Updated: src/coverage/* to drop zero_config_manager imports and unified discovery to gcov path.
• Updated: src/config/core/config_defaults_core.f90 to remove unused zero_config_manager use.
• Added: scripts/fpm_coverage_bridge.sh to run tests (fpm/ctest) and generate .gcov files with gcov.

Verification

• Baseline and post-change tests with fpm test pass locally (120s timeout enforced). Key outputs include:
  • OK: markdown minimal
  • OK: cli flags minimal and rejections (incl. --zero-config rejected)
  • OK: gcov processing smoke
• Manual: fortcov --discover-and-gcov now invokes the bridge (non-fatal), then proceeds with gcov discovery and Markdown generation. Manual mode unchanged.

Rationale

• Simplifies code by removing brittle Fortran zero-config orchestration. Consolidates to a single, explicit auto path via a small shell bridge, aligning with repository direction and reducing maintenance.