Classify Pi edit operations as file updates

[bb-slop-cop]

Human comments

What was wrong

The Pi delta translator handled edit and write operations in one branch and inferred the file-change kind only from an optional top-level prior-text field. Pi edit batches use nested replacement entries, so the translator emitted an addition for an edit. The behavior and root cause are documented in the reproduction report.

What changed

Pi edit operations now emit update file changes regardless of argument shape. Existing content-only write operations remain additions, and legacy operations with a top-level prior-text field retain their existing update behavior. A focused translator regression covers the nested edit-batch shape.

How you verified

• The new regression failed on unchanged origin/main: expected update, received add.
• The focused regression passed after the fix.
• pnpm exec turbo run test typecheck --filter=bb-plugin-provider-pi: 123 tests passed and typecheck passed.
• pnpm exec oxfmt --check plugins/provider-pi/src/delta-translation.ts plugins/provider-pi/src/delta-translation.test.ts: passed.
• git diff --check: passed.
• Changed-line count: 30 text lines (29 additions, 1 deletion).

Fixes #3240

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