Add go-deep validation mode

[sentry-junior]

add a go-deep mode to the harness so each finding gets a second, higher-reasoning validation pass.

• run a fuller upstream/downstream code trace before final severity
• use that pass to confirm, downgrade, or disprove findings
• treat disproven cases as defense-in-depth or non-issues instead of inflated severity
• reuse the claude sdk path so the normal tool suite stays available

Action taken on behalf of David Cramer.