fix(core): reuse shared patch diff

[kitlangton]

What

Consolidate Patch file-diff construction on the shared fileDiff helper used by edit/write. This corrects the observable deletion count for a one-line file ending in a newline from 2 to 1.

Before / After

Before: deleting a file containing remove\n generated a patch with one removed line but returned deletions: 2 because Patch counted before.split("\n").length. Patch also duplicated patch construction and line-counting logic already owned by fileDiff.

After: Patch delegates patch construction and additions/deletions counting to fileDiff, so both remove\n and remove report deletions: 1. Move results still expose the destination resource in file, while generated patch headers still identify the source path.

How

• packages/core/src/tool/plugin/patch.ts calls fileDiff with the source absolute path and Patch status, then preserves Patch-specific destination-resource projection and trimDiff processing.
• packages/core/test/tool-patch.test.ts covers one-line deletions with and without a trailing newline and pins move patch headers to the source path.

Scope

• No changes to the shared fileDiff helper.
• No changes to Patch formatting, mutation, permission, or move behavior beyond consolidating diff construction and correcting the derived deletion count.

Testing

• cd packages/core && bun run test test/tool-patch.test.ts (37 passed)
• cd packages/core && bun typecheck
• bun run lint packages/core/src/tool/plugin/patch.ts packages/core/test/tool-patch.test.ts
• bunx prettier --check packages/core/src/tool/plugin/patch.ts packages/core/test/tool-patch.test.ts
• bunx ast-grep scan -c script/ast-grep/sgconfig.yml packages/core/src/tool/plugin/patch.ts
• Push hook: bun turbo typecheck --concurrency=3 (33 tasks passed)

Also attempted the repository-wide bun run lint:effect-patterns; it remains blocked by six pre-existing diagnostics in unrelated files (filesystem/watcher.ts, session.ts, aisdk.ts, models-dev.ts, tool/read-filesystem.ts, and tool/runtime.ts).