Fix ccache permission denied on fresh devcontainer volumes

[ltowarek]

Summary

• Pre-create /home/vscode/.ccache with correct ownership (vscode:vscode) in the devcontainer Dockerfile, using the same mkdir -p + chown pattern already applied to .claude

Motivation

Docker initialises named volumes with root:root ownership on the mount-point directory. When the ccache volume is mounted for the first time, Docker copies the image layer at that path into the empty volume — preserving file ownership. Without the pre-created directory in the image, the volume is initialised as root-owned and the vscode user gets Permission denied when ccache tries to create its tmp/ subdirectory.

Test plan

• Delete the existing ccache Docker volume (docker volume rm ccache) and reopen the devcontainer
• Run idf.py build — ccache should operate without permission errors

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