Add doctor check for sandbox Go cache size

[yottadynamics]

Problem or motivation

Sandboxed Go test runs use an isolated persistent cache under ~/.yottacode/sandbox-go-cache/ so containers can reuse GOCACHE and GOMODCACHE without bind-mounting the host's normal Go cache paths.

That isolation is the right default, but the cache is currently hidden and unbounded. On one local machine it reached:

du -csh ~/.yottacode/sandbox-go-cache/
2.2G    /home/ppetkov/.yottacode/sandbox-go-cache/
2.2G    total

A quick breakdown showed most of that was compiled build/test cache, not downloaded modules:

1.7G    ~/.yottacode/sandbox-go-cache/cache
479M    ~/.yottacode/sandbox-go-cache/modcache

Proposed solution

Add a yottacode doctor check that reports sandbox Go cache size when ~/.yottacode/sandbox-go-cache/ exists, with a warning when it exceeds a reasonable threshold.

The check should distinguish at least:

• total sandbox Go cache size
• build/test cache size: ~/.yottacode/sandbox-go-cache/cache
• module cache size: ~/.yottacode/sandbox-go-cache/modcache

Suggested behavior:

• no warning when the directory does not exist
• informational output when it exists and is modest
• warning when total size exceeds a threshold such as 1–2 GB
• include cleanup guidance that preserves the distinction between build cache and module cache

Example guidance:

# Reclaim compiled build/test artifacts; modules remain cached.
GOCACHE="$HOME/.yottacode/sandbox-go-cache/cache" \
GOMODCACHE="$HOME/.yottacode/sandbox-go-cache/modcache" \
go clean -cache -testcache

# Stronger cleanup; next sandboxed Go run may redownload dependencies.
GOCACHE="$HOME/.yottacode/sandbox-go-cache/cache" \
GOMODCACHE="$HOME/.yottacode/sandbox-go-cache/modcache" \
go clean -cache -testcache -modcache

Alternatives considered

• Reuse host Go cache paths directly (~/.cache/go-build, ~/go/pkg/mod, or custom host GOCACHE/GOMODCACHE). Rejected as the default because it widens the sandbox boundary, exposes/mutates broader developer state, and complicates Podman/SELinux/cache configuration.
• Automatically clean on every sandboxed test run. Rejected because it defeats cache reuse and would make the first Go command in every sandbox session slow again.
• Automatically clean modcache. Rejected as a default because it causes future dependency downloads.

Prior art

Go exposes explicit cleanup through go clean -cache, go clean -testcache, and go clean -modcache. yottacode should surface the same distinction for its own isolated sandbox cache.

Would you be willing to contribute this?

Yes, I'd like to open a PR.