Cover more secret shapes in the canonical .gitignore (closes #38)

The canonical .gitignore matched only .env, .env.*, *.pem and *.key, so prod.env, .envrc, *.p12 and *.pfx bundles, and the SSH private keys id_rsa, id_dsa, id_ecdsa and id_ed25519 could be committed. The secrets section now covers the same shapes as the canonical .dockerignore, written to gitignore's own rules: unanchored, no **/ prefix, character ranges for case. example.env and sample.env stay trackable through negations, and the comment tells a repository to add its own negation for any other committed template.

Judgement call: the existing entries were rewritten with character ranges, which only widens them, and the bare .env line is dropped because *.env covers it.

Model: opus-5-5

.gitignore

@@ -20,8 +20,26 @@ 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`.
+
+# 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][dD]25519

TODO.md

@@ -21,6 +21,11 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-10-03: Brought the canonical `.gitignore` level with `.dockerignore` on
+  secrets (issue 38): it now also ignores `prod.env`-style `*.env` files,
+  `.envrc`, `*.p12`, `*.pfx` and the extensionless SSH private keys, written to
+  `.gitignore`'s own rules (no `**/` prefix) and case-folded with character
+  ranges. `example.env` and `sample.env` stay trackable through negations.
 - 2026-10-02: The image version now comes from git inside the build (issues 69
   and 71), superseding the 2026-09-08 entry that excluded `.git`. The canonical
   `.dockerignore` sends `.git` but keeps out `.git/config`, which can hold a