Ask for missing credentials and create the project's own .env

.env becomes optional. A credential it does not provide (an absent file, an
absent or empty key) is asked, without echo: GITEA_TOKEN at the start,
GITHUB_PAT and GITHUB_USER once GitHub is chosen. An invalid value is asked
again and never shown; when input ends the run stops before any request.
Asked tokens are registered for redaction at once.

After the local project exists the script asks (default no) whether to
create a .env in it. On a yes it holds only the needed keys, is created
private (mode 600) from the start, is excluded from git through
.git/info/exclude (no tracked file changes), is never replaced without a
second yes and is never written when git tracks it. The summary names the
keys, never the values.

New library files credentials.sh and envfile.sh; README, .env.example and
the security decisions updated; tests cover every case.

Task: MIL-005#1
Task: MIL-005#2
Task: MIL-005#3
Task: MIL-005#4
Task: MIL-005#5
Closes #35
Closes #36
Closes #37
Closes #38
Closes #39

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/lib/plan.sh

@@ -54,4 +54,5 @@ print_plan() {
   else
     say "$(printf '  %-18s: %s' "Framework" "NOT possible without SSH to Gitea; you will be asked whether to go on without it")"
   fi
+  say "$(printf '  %-18s: %s' "Project .env" "you are asked whether to create it ($(env_file_key_list))")"
 }