fix(xlsx): exclude phonetic guide text from cell values

[y4ho0]

Summary

• exclude <rPh> phonetic-guide text from ordinary XLSX cell display values
• preserve direct shared-string text and rich-text runs
• keep the guide available separately through the existing format.phonetic readback

Fixes #343

Validation

Using the reporter's attached sample.xlsx:

officecli view sample.xlsx html
# Before: <td data-path="/Sheet1/A1">項目コウモク</td>
# After:  <td data-path="/Sheet1/A1">項目</td>

officecli view sample.xlsx text
# Before: A1=項目コウモク
# After:  A1=項目

officecli get sample.xlsx /Sheet1/A1 --json
# text: "項目"
# format.phonetic: "コウモク"

officecli validate sample.xlsx
# Validation passed: no errors found.

I also created a shared-string cell with two formatted runs (Hello bold, world italic). view text still returns Hello world, view html retains both formatted spans, and officecli validate passes.

Project build:

dotnet build src/officecli/officecli.csproj -c Release --no-restore
# Build succeeded (one pre-existing CS8602 warning in ExcelHandler.SheetShift.cs).