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

README.md

@@ -103,6 +103,7 @@ A detail that is set is used and not asked; the summary marks it with
 | `GITHUB_OWNER` | GitHub user or organization | letters, digits, `-`; used only when GitHub is used |
 | `PROJECT_DIRECTORY` | local directory | not empty, not starting with `-` |
 | `ENABLE_PLAN_GATE` | enable the plan gate | `yes` or `no` |
+| `PROJECT_LICENSE` | license of the project | a Gitea license key (letters, digits, `.`, `+`, `-`; at most 64), such as `AGPL-3.0` or `MIT`, or `none` |
 
 - A key that is present counts as set, even when its value is empty. Only
   `PROJECT_DESCRIPTION` may be empty (no description); an empty value for any
@@ -111,7 +112,12 @@ A detail that is set is used and not asked; the summary marks it with
   key. The script never falls back to asking for it.
 - `USE_GITHUB=no` skips the GitHub owner and every GitHub step; a
   `GITHUB_OWNER` set at the same time is ignored, with a warning.
-- Only these eight details can be set. The confirmations stay questions that
+- `PROJECT_LICENSE` is never asked. When set, that license is put on the Gitea
+  repository with or without GitHub, and `none` means no license. When absent,
+  AGPL-3.0 is applied only if GitHub is used **and** the project is public;
+  a private project, or one without GitHub, gets no license. The Gitea server
+  must offer the license, or the run stops before anything is created.
+- Only these nine details can be set. The confirmations stay questions that
   default to no: create now, reusing an existing repository, an existing
   directory, `core.hooksPath` and replacing a template file.
 - These keys are accepted in `config.env` only, never in `.env`.
@@ -119,7 +125,7 @@ A detail that is set is used and not asked; the summary marks it with
   value there. Put a description that contains ` #` in double quotes, for
   example `PROJECT_DESCRIPTION="Tool for #mirrors"`.
 
-With all eight set, a run asks only the confirmations:
+With all of them set, a run asks only the confirmations:
 
 ```bash
 src/create-project.sh --apply        # asks only "Create these now (y/n) [n]"
@@ -153,8 +159,8 @@ with read-only requests and prints a plan:
 
 ```text
 Plan:
-  Gitea repository  : create (private) with the AGPL-3.0 license https://git.example.org/Team/my-app
-  GitHub repository : create (private), empty https://github.com/acme/my-app
+  Gitea repository  : create (public) with the AGPL-3.0 license (default: GitHub and a public project) https://git.example.org/Team/my-app
+  GitHub repository : create (public), empty https://github.com/acme/my-app
   Push mirror       : Gitea -> GitHub every 10m0s
   Local project     : create ./my-app (new directory), git on main, no commit
   Local origin      : will use SSH (the SSH test passed)
@@ -167,7 +173,7 @@ Without `--apply` that is all that happens. With `--apply` the script asks
 "Create these now" (default no) and then creates, in this order:
 
 1. the GitHub repository (empty), if chosen;
-2. the Gitea repository (with the AGPL-3.0 license if GitHub was chosen);
+2. the Gitea repository (with the license that applies: `PROJECT_LICENSE`, or AGPL-3.0 for a public project with GitHub);
 3. the push mirror Gitea -> GitHub, and a request for its first sync;
 4. the local directory, `git init` on `main`, the `origin` remote (and `github`
    if chosen), and, if the Gitea repository holds the license commit, that
@@ -181,10 +187,11 @@ hooks refuse commits on `main`.
 
 ### Choices
 
-- **GitHub or not.** Choosing GitHub also applies the AGPL-3.0 license to the
-  Gitea repository (so it is not empty) and sets up the mirror. Without GitHub
-  the Gitea repository is empty and has no license, and `GITHUB_PAT` is not
-  needed.
+- **GitHub or not.** Choosing GitHub sets up the mirror. For a public project it
+  also applies the AGPL-3.0 license to the Gitea repository (so it is not
+  empty), unless `PROJECT_LICENSE` says otherwise. A private project with
+  GitHub gets no license by default. Without GitHub the Gitea repository has
+  no license unless `PROJECT_LICENSE` sets one, and `GITHUB_PAT` is not needed.
 - **Owners.** The Gitea owner and the GitHub owner are chosen separately and
   may be a user or an organization. `GITHUB_USER` is only the suggested default
   for the GitHub owner prompt; it identifies who authenticates.