write.ts: no pre-read check before writing, silently overwrites existing files #30864

Description

File: packages/core/src/tool/write.ts:67

return yield* files.writeTextPreservingBom({ plan, content: parameters.content })

No pre-existence check. After compaction/pruning (long sessions), AI loses memory of file contents and may write to an existing file, silently overwriting user code.

V1 had a create tool with flag: "wx" that rejected existing files, and a separate write tool. V2 merged them into one write tool that overwrites unconditionally.

Fix: Add a pre-read guard that fails the tool when the file already exists, forcing the AI to use edit instead.

OpenCode version

v1.16.0 (commit 3cf1cef)