Re-vendor the canonical files from sneak/prompts at dd4027b (closes #171)

.dockerignore, .gitignore, .prettierignore and REPO_POLICIES.md are
copied from sneak/prompts at dd4027b; .editorconfig, check.yml and
.prettierrc already matched. This repository's build artifacts follow
the copied content in .gitignore and, anchored, in .dockerignore. The
scripts follow the new policy: check runs fmt-check, cibuild bootstraps
and runs check before the image build, fmt and fmt-check find the
pinned yarn under nvm, and the image's last stage marks /app safe for
git. The build-context test expects the new pattern forms.

Judgement call: script/precommit runs lint and fmt-check without the tests, so the README's red-phase test commit can still land; the policy allows that form only where local testing is impossible.
Not applicable: the issue's .golangci.yml entries; this TypeScript repository has none.

Model: opus-5-5

.gitignore

@@ -11,9 +11,41 @@ Thumbs.db
 .vscode/
 *.sublime-*
 
+# Agent scratch (worktrees of this repo, created and destroyed by
+# in-flight tooling). Unanchored: .gitignore patterns already match at
+# every depth, so no prefix is wanted here. This is not a .dockerignore
+# entry and must not be given a `**/` prefix on the way into one.
+.claude/
+
 # Node
 node_modules/
 
+# Secrets. Unanchored like every entry above, so each matches at every
+# depth. Matching is case-sensitive on Linux, so names use character
+# ranges rather than a lowercase form that misses `Server.Key`.
+
+# Environment files. `*.env` covers bare `.env` and the `prod.env`
+# convention. Only the templates `example.env` and `sample.env` are
+# re-included below. A repository that commits any other template adds
+# its own negation after these lines, for example `!.env.example`.
+*.[eE][nN][vV]
+.[eE][nN][vV].*
+.[eE][nN][vV][rR][cC]
+!example.env
+!sample.env
+
+# Private keys and the bundles carrying them.
+*.[pP][eE][mM]
+*.[kK][eE][yY]
+*.[pP]12
+*.[pP][fF][xX]
+[iI][dD]_[rR][sS][aA]
+[iI][dD]_[dD][sS][aA]
+[iI][dD]_[eE][cC][dD][sS][aA]
+[iI][dD]_[eE][cC][dD][sS][aA]_[sS][kK]
+[iI][dD]_[eE][dD]25519
+[iI][dD]_[eE][dD]25519_[sS][kK]
+
 # TypeScript / build artifacts
 dist/
 build/
@@ -24,18 +56,8 @@ coverage/
 # Vitest
 .vitest-cache/
 
-# Environment / secrets
-.env
-.env.*
-*.pem
-*.key
-
 # Compiled binary (built by make build-bin)
 bin/quak
 
 # quak runtime data (in case anyone runs the CLI from inside the repo)
 .quak/
-
-# Local per-developer tool settings and scratch state, including the
-# worktrees agents check out under this directory
-.claude/