Fold ScriptRewriter into RewriteBootstrapScriptsStep #573

Problem

src/Fallout.Migrate/Steps/ mixes two shapes. Most files are migration steps that implement IMigrationStep. ScriptRewriter is a free-floating static class that is not a step — it only holds the rewrite rules for one step.

That split was the original design in #507, but it has since been retired one file at a time:

• CsprojRewriter was folded into RewriteCsprojsStep in aef6c073, on the grounds that it had a single caller.
• CodeRewriter was folded into RewriteCsFilesStep in Smooth the Fallout.Common.ProjectModel → Fallout.Solutions migration path #528, for the same reason.

ScriptRewriter is the last one. It has exactly one caller, RewriteBootstrapScriptsStep.cs:26, so the same reasoning applies. Until it moves, Steps/ teaches two contradictory patterns, and a contributor adding a bootstrap-script rewrite rule has to guess which one to follow.

Outcome

Every file in src/Fallout.Migrate/Steps/ is a migration step, and each step owns the rewrite rules for its own artifact type.

Acceptance criteria

• ScriptRewriters regex fields and Rewrite method live in RewriteBootstrapScriptsStep
• src/Fallout.Migrate/Steps/ScriptRewriter.cs is deleted
• ScriptRewriterSpecs is renamed to RewriteBootstrapScriptsStepSpecs, file and class
• dotnet test tests/Fallout.Migrate.Specs passes with no test changes beyond the rename
• No behaviour change

Notes

Good first issue: a mechanical move of about 50 lines with no design decisions. Copy the shape of aef6c073 (Fold CsprojRewriter into RewriteCsprojsStep) — it is the same change on a sibling step. The convention is written up in #574.