Set the project license in config.env; AGPL-3.0 default only for a public GitHub project

- PROJECT_LICENSE (a Gitea license key, or none) is read, checked and never asked
- Without it AGPL-3.0 applies only when GitHub is chosen and the project is public
- The license is applied on Gitea with or without GitHub, and checked against the server first
- Plan and summary name the license and where it came from
- MIL-006 accepted; README and config.env.example document the key
- Tests: new test-license.sh; existing tests use a public project where they expect AGPL-3.0

Task: MIL-006#1
Task: MIL-006#2
Task: MIL-006#3
Task: MIL-006#4
Refs #44
Refs #45
Refs #46
Refs #50

src/lib/config.sh

@@ -142,6 +142,7 @@ validate_project_presets() {
   check_preset GITHUB_OWNER is_valid_github_owner "$HINT_GITHUB_OWNER"
   check_preset PROJECT_DIRECTORY is_valid_directory "$HINT_DIRECTORY"
   check_preset_choice ENABLE_PLAN_GATE yes no
+  check_preset PROJECT_LICENSE is_valid_license "$HINT_LICENSE"
 }
 
 validate_credentials() {