Re-vendor the canonical files from sneak/prompts at dd4027b (closes #65)

The vendored files are fetched from sneak/prompts commit dd4027b, with
this repository's /bin carried forward in .dockerignore; the
test-support deny list has no entries of its own. The lint phase moves
to golangci-lint v2.14.0. The build stage now takes the version from
git describe on the .git the build context carries, unless VERSION is
passed, and fails when .git is present but no version comes out. The
test phase drops -count=1, which the policy says it does not need, and
keeps its tmpfs build cache. One test calls Header.Get with X-Real-IP,
as canonicalheader asks.

Deviation: the Go lines of .gitignore and .editorconfig are dropped;
they are not exempt from byte-identity.

Model: opus-5-5

.gitignore

@@ -20,14 +20,28 @@ Thumbs.db
 # 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: the binary `make build` writes, test binaries, profiles and logs
-/bin/
-*.test
-*.out
-*.log
+# 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]