Re-vendor the canonical files from sneak/prompts at dd4027b (closes #95)

The shared files are the copies at sneak/prompts commit dd4027b, with this
repository's own entries kept after them. script/lint, script/test and
REPO_POLICIES.md come from its next at c55a0cb, so the lint and test
builds write no image. golangci-lint is v2.14.0 and raises no findings.
Lint and test are phases of the Dockerfile; the tests run under the race
detector as nobody, so Dockerfile.lint, script/verify-lint-image-pin and
make test-race are gone. Every docker build in script/ passes --no-cache.
Formatting runs on the host: script/bootstrap installs the pinned node and
yarn, and the prettier and markdown stages are gone. .claude/settings.json
is deleted.

Deviation: the workflow keeps fetch-depth: 0.
Deviation: .gitignore keeps the scan database patterns.
Over the cap: make test takes 82 to 100 seconds on this host.

Model: opus-5-5

.gitignore

@@ -11,25 +11,48 @@ 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/
 
-# Environment / secrets
-.env
-.env.*
-*.pem
-*.key
+# 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`.
 
-# Go build artifacts
-/sfdupes
-*.test
-*.out
+# 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]
+
+# This repository's own entries, kept after the canonical content above.
 *.log
+*.out
+*.test
+/sfdupes
 
-# Local scan data
+# A scan database lists every path it scanned.
 *.sqlite
 *.sqlite-shm
 *.sqlite-wal
-
-# Agent worktrees
-.claude/worktrees/