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

README.md

@@ -706,9 +706,9 @@ provide:
   install-precommit)
 - `script/projectname` — output the project name ("pixa")
 - `script/test` — run the test suite: build the `test` phase of the
-  `Dockerfile`, tagged `pixa-test`
+  `Dockerfile`, writing no image
 - `script/lint` — run golangci-lint: build the `lint` phase of the `Dockerfile`,
-  tagged `pixa-lint`; the linter never runs on the host
+  writing no image; the linter never runs on the host
 - `script/fmt` — format the Go code with gofmt and the markdown with prettier
   (writes)
 - `script/fmt-check` — check the same formatting (read-only), on the host
@@ -726,15 +726,20 @@ provide:
 - `script/install-precommit` — install the git pre-commit hook that runs
   `script/precommit`
 
-Every `docker build` in these scripts passes `--no-cache`, so the lint and test
-phases run on every build instead of coming from the build cache.
+Every `docker build` in these scripts passes a new value of the `CACHEBUST`
+build argument, which every stage of the `Dockerfile` declares first, so every
+`RUN` step, the lint and test phases included, runs on every build instead of
+coming from the build cache. Go's build cache is kept from build to build: the
+steps that compile mount it, so a package whose inputs are unchanged is not
+compiled again, and both `go test` runs pass `-count=1` so that no stored test
+result is replayed. The `test` phase runs on the Debian Go image, with Debian's
+libvips and libheif; the image pixa ships is built and runs on Alpine.
 `script/check`, `script/cibuild`, `script/docker`, `script/lint`, `script/test`,
 `script/setup` and `script/install-precommit` are the standard copies from
 `sneak/prompts`, kept identical to them. `script/fmt` and `script/fmt-check` are
 the standard copies with pixa's `gofmt` step kept before prettier. prettier
-formats the markdown only: not the HTML templates, as it cannot parse a Go
-template action inside a tag, and not `REPO_POLICIES.md` (see
-`.prettierignore`), a copy of the one in `sneak/prompts`.
+formats the markdown only, not the HTML templates, as it cannot parse a Go
+template action inside a tag.
 
 ## Load Test