Re-vendor the canonical files from sneak/prompts at e3f79c4 (closes #221)

The vendored dotfiles, REPO_POLICIES.md and the standard scripts are
fetched from sneak/prompts commit e3f79c4, with pixa's own entries kept
after the standard part. The scripts pass a new CACHEBUST build argument
in place of --no-cache and every Dockerfile stage declares it first, so
every RUN step reruns while Go's build cache, mounted on the steps that
compile, is kept. The lint phase runs golangci-lint v2.14.0, the test
phase runs on the Debian Go image, and the build stage takes the version
as the policy gives it. The two new lint findings are fixed in the code.

Model: opus-5-5

internal/imgcache/cachesize.go

@@ -78,10 +78,6 @@ func (c *Cache) computeDefaultMaxBytes(
 	computed := spaceBytes / freeSpaceFractionDenominator * freeSpaceFractionNumerator
 	computed = min(computed, math.MaxInt64)
 
-	// gosec cannot see that min() above bounds computed, so it reads
-	// this conversion as potentially overflowing. It cannot: computed is
-	// at most math.MaxInt64 on every path here.
-	//nolint:gosec // G115: clamped to MaxInt64 by min above
 	limit := int64(computed)
 	limit = max(limit, DefaultCacheMaxBytesFloor)