[Bug]: Malformed config gets silently overwritten on startup

[ben-ranford]

Bug Description

Malformed configuration files are silently overwritten during startup paths. If ~/.ggcconfig.yaml contains invalid YAML, ggc logs a parse error but then rewrites the file with generated defaults.

Steps to Reproduce

1. Create a malformed config file:

   MOD=$(go env GOMODCACHE); CCH=$(go env GOCACHE); TMP=$(mktemp -d)
   printf 'broken: [yaml\n' > "$TMP/.ggcconfig.yaml"

2. Run ggc with that HOME:

   HOME="$TMP" GOMODCACHE="$MOD" GOCACHE="$CCH" go run . version >/dev/null 2>/tmp/err.txt

3. Inspect stderr and config file:

   head -n1 /tmp/err.txt
   sed -n '1,6p' "$TMP/.ggcconfig.yaml"

Expected Behavior

If config parsing fails, ggc should report the error and leave the existing config file untouched.

Screenshots

N/A

Environment

• OS: macOS 26.2
• Go Version: go version go1.25.7 darwin/arm64
• ggc Version: ggc version v1.0.0 (commit: abc123)
• git Version (run git --version): git version 2.39.5 (Apple Git-154)

Additional Context

Root cause was two startup write paths after parse failure:

1. pkg/config.Manager.LoadConfig() always called Save().
2. cmd/version proceeded with metadata updates that call Set() (which persists config).

closes #343