Re-vendor the canonical files from sneak/prompts at dd4027b (closes #121)

The vendored files are copies from sneak/prompts dd4027b, with this
repository's own entries after the canonical content. golangci-lint is
v2.14.0. Lint and test are phases of the Dockerfile, which script/lint
and script/test build with --no-cache; Dockerfile.lint and
script/lint-darwin are gone, and the lint phase also checks the macOS
build. The tests run on the Debian Go image with cgo and the race
detector. script/cibuild bootstraps, runs script/check and builds the
image; CHECK_EPOCH and the memlock ulimit are gone. Go's build cache
stays in a cache mount, out of the test image's layer. Agent guidance
lives in AGENTS.md alone; the tool-specific file is gone.

Model: opus-5-5

AGENTS.md

@@ -100,6 +100,18 @@ Version: 2025-06-08
     are acceptable in the root, but source code and other files should be
     organized in appropriate subdirectories.
 
+13. Commit messages carry no author or co-author attribution for the coding
+    agent. The agent is an inanimate tool and the owner is the sole author of
+    the code it writes; such a line is advertising, not attribution.
+
+14. Never run part of the test suite: always run the whole thing with
+    `make test`.
+
+15. Do not stop working on a task until you have reached the definition of done
+    it gives. Do all of the work, not part or most of it.
+
+16. After each commit, push to the remote.
+
 ## Python-Specific Guidelines
 
 1. **Type Annotations (UP006)**: Use built-in collection types directly for type
@@ -154,10 +166,17 @@ Version: 2025-06-08
 
 3. **Wrap errors** with `fmt.Errorf("context: %w", err)` for debuggability.
 
-4. **Never modify linter config** (`.golangci.yml`) to suppress findings. Fix
-   the code.
+4. **Never modify linter config** (`.golangci.yml`) to suppress findings, and do
+   not modify it at all unless specifically instructed. Fix the code.
 
 5. **All PRs must pass `make check` with zero failures.** No exceptions, no
    "pre-existing issue" excuses.
 
 6. **Pin external dependencies by commit hash**, not mutable tags.
+
+7. **A program's `main.go` is `./cmd/<program_name>/main.go`** and only imports
+   and calls `<program_name>.CLIEntry()`; the implementation is in
+   `./internal/<program_name>/`. This keeps several programs in one repository
+   from cluttering the root directory.
+
+8. **Log with `log/slog`.**