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

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