Strip legacy NUKE_ENTERPRISE_TOKEN blocks from bootstrap scripts during migration #522

Problem

ScriptRewriter (used by RewriteBootstrapScriptsStep) rewrites build.cmd/build.ps1/build.sh but leaves behind legacy NUKE_ENTERPRISE_TOKEN feed-registration blocks (the nuke-enterprise NuGet source add/remove lines), which were already dead in NUKE. Migrated repos carry this dead code forward.

Outcome

fallout-migrate strips these obsolete NUKE_ENTERPRISE_TOKEN blocks from bootstrap scripts during rewrite, instead of preserving them.

Acceptance criteria

• ScriptRewriter removes the NUKE_ENTERPRISE_TOKEN conditional block (both .ps1 and .sh variants) when present
• Existing bootstrap scripts without this block are unaffected

Notes

From #509 (review comment).