fix(tools): make file write outcomes reliable

[elliottophellia]

Summary

Make write_file outcomes explicit and reliable across tool execution, persisted chat history, and the project Changes sidebar.

• Reject missing content before any filesystem mutation while allowing intentional zero-byte files.
• Reject directory targets with an actionable error.
• Report empty files as 0 bytes, 0 lines.
• Instruct the agent not to claim creation until write_file returns successfully.
• Persist and restore tool error state so failed writes remain visibly failed after reload.
• Render persisted tool calls without a result as incomplete, never as successful writes.
• Include only completed, successful write_file and edit_file calls in the project Changes sidebar.
• Add focused backend and frontend regression coverage.

Verification

• go test ./...
• bun test
• bun x tsc -b --noEmit
• make build
• Browser smoke against the built dashboard on http://127.0.0.1:8787

Manual Checks

• A directory target renders an actionable failed write and remains failed after reload.
• An intentional empty file is created as a real zero-byte file.
• An empty overwrite truncates an existing file successfully.
• Failed and interrupted writes are not listed as project changes.