docs: normalize Rails 7 migration pattern docs (WA-DOC-018)

[kitcommerce]

Summary

Normalizes all docs in docs/rails7-migration-patterns/ to a consistent five-section structure and updates the README index.

Closes #909 (WA-DOC-018)

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Changes

Section normalization (all 11 pattern docs)

Each pattern doc now has these canonical ## sections in order:

1. Symptom — observable problem
2. Root cause — why it happens
3. Detection — how to identify if you're affected
4. Fix — how to resolve it
5. References / Links — upstream docs, related issues

Heading renames

• Root Cause → Root cause (assets-pipeline, bigdecimal, sprockets-manifest)
• See Also → References (assets-pipeline)
• Workarea PR (or Issue) → References / Links (url-routing-helpers)
• Workarea PR / Issue → References (middleware-stack-ordering, sprockets-manifest)
• Migration Steps → Fix (webpacker-to-sprockets-4)

New sections added

Docs that were status reports / audit notes rather than structured patterns received
the full Symptom/Root cause/Detection/Fix preamble:

• webpacker-to-sprockets-4.md
• jsbundling-rails-esbuild.md
• deprecation-sweep-results.md
• error-reporting.md
• load-defaults-7-2.md
• rails-7-2-notes.md
• zeitwerk-notes.md

README index

Replaced the partial bullet list with a full Markdown table linking all 11 pattern docs
with one-line descriptions.

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Client impact

None — documentation changes only.

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Verification Plan

1. Confirm each of the 11 pattern docs has all five required ## sections:

   for f in docs/rails7-migration-patterns/*.md; do
     echo "=== $(basename $f) ==="
     grep "^## " "$f" | grep -iE "symptom|root cause|detection|fix|references"
   done

   Expected: each non-README file prints all five sections.

2. Confirm README links resolve — all filenames in the table exist:

   ls docs/rails7-migration-patterns/

3. Confirm no hardcoded local paths remain:

   grep -rn 'Users/Shared/openclaw' docs/rails7-migration-patterns/

   Expected: no output.