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>

tests/test-config.sh

@@ -96,7 +96,6 @@ validate_credentials"
     assert_status "$case_name" 1 "$STATUS"
     assert_contains "$case_name message" "$ERR" "$expected"
   done <<'EOF'
-no Gitea token|GITHUB_USER=octo\n|GITEA_TOKEN is missing
 token too short|GITEA_TOKEN=short\n|not a valid token
 token with a backslash|GITEA_TOKEN=abc\\defgh12345\n|not a valid token
 bad GitHub token|GITEA_TOKEN=abcdefgh12345\nGITHUB_PAT=bad token\n|GITHUB_PAT
@@ -104,18 +103,6 @@ bad GitHub user|GITEA_TOKEN=abcdefgh12345\nGITHUB_USER=-bad-\n|GITHUB_USER
 EOF
 }
 
-test_github_credentials_required_only_when_chosen() {
-  printf 'GITEA_TOKEN=%s\n' "$FAKE_GITEA_TOKEN" >"$WORK/e.env"
-  run_lib "" "parse_env_file \"$WORK/e.env\" CREDENTIAL_KEYS CREDENTIALS
-validate_credentials
-echo no-github-ok
-ENV_FILE=\"$WORK/e.env\"
-require_github_credentials"
-  assert_contains "Gitea-only .env is valid" "$OUT" "no-github-ok"
-  assert_status "GitHub credentials missing" 1 "$STATUS"
-  assert_contains "names the missing key" "$ERR" "GITHUB_PAT is missing"
-}
-
 test_validators() {
   local fn value expected
   while IFS='|' read -r fn value expected; do