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

The dotfiles, REPO_POLICIES.md and the standard scripts are fetched
whole from sneak/prompts e3f79c4, with pixa's own entries kept after
them. Check builds pass a new CACHEBUST build argument instead of
--no-cache, and every Dockerfile stage declares it first, so every RUN
step still runs while Go's build cache is kept. Lint runs golangci-lint
v2.14.0, and the test phase runs on the Debian Go image. Of the two new
lint findings, one is fixed and one (gosec G703 on the fixed config file
locations) is suppressed. The agent guidance moves unchanged from
CLAUDE.md to AGENTS.md.

Deviation: the workflow keeps its docker-smoke step while issue 203
waits on the owner.

Model: opus-5-5

.editorconfig

@@ -13,3 +13,6 @@ indent_style = tab
 
 [*.go]
 indent_style = tab
+
+# This repository's own sections, such as one for another language it
+# uses, go below this comment, and a re-vendor keeps them.