fix: use conventional exit codes for reports

[ehsanestaji]

Summary

• return process exit 0 whenever FastaGuard successfully writes requested reports, regardless of PASS/WARN/FAIL QC status
• keep QC decisions in report fields such as verdict.status, gate.status, and gate.blocking_findings
• simplify local Snakemake and nf-core starter examples by removing captured exit-code workarounds

Why

This aligns FastaGuard with standard CLI/workflow expectations raised during Snakemake and nf-core wrapper review: process exit should indicate tool execution success/failure, while QC gating should be handled from machine-readable report fields.

Verification

• cargo fmt --all --check
• cargo test --locked --all-targets
• cargo clippy --locked --all-targets -- -D warnings
• python3 -m unittest discover -s tests/python

Follow-up after merge/release

• publish a patch release before updating external workflow PRs
• simplify the Snakemake wrapper PR
• simplify the nf-core module PR