fix: suppress Sigstore certificate payload false positives

[koriyoshi2041]

Summary

• mask only certificate rawBytes inside structurally valid Sigstore v0.3 skill.oms.sig bundles before static pattern matching
• preserve byte-for-character offsets so findings in every other bundle field keep correct locations
• keep malformed bundles, differently named files, and non-certificate payloads fully visible to analyzers

Why this approach

Signed skills currently receive SC3 and MP2 findings from base64-encoded DER certificates. Skipping the whole signature file would create a forgeable analysis bypass; this instead implements the narrow field-level suppression suggested in the review of #261. Unknown fields and attacker-controlled content remain scanned.

Closes #336.

Validation

• uv run pytest tests/nodes/analyzers/test_static_runner_filtering.py tests/nodes/analyzers/test_static_patterns.py tests/nodes/analyzers/test_mp2_regex_backtracking.py -q — 139 passed
• uv run ruff check src/skillspector/nodes/analyzers/static_runner.py tests/nodes/analyzers/test_static_runner_filtering.py
• uv run ruff format --check src/skillspector/nodes/analyzers/static_runner.py tests/nodes/analyzers/test_static_runner_filtering.py
• git diff --check

Risk

The exemption is limited to the exact certificate chain path in a valid v0.3 bundle named skill.oms.sig. It does not verify the signature or attest trust; it only prevents opaque certificate bytes from reaching generic content heuristics.