Introduce component-based licensing: Serena GPL-3.0-or-later, SolidLSP MIT

The repository is licensed per component. SolidLSP (src/solidlsp,
test/solidlsp, test/resources) remains MIT-licensed and independently
reusable; the Serena application (src/serena, src/interprompt, scripts,
test/serena, docs) is licensed under GPL-3.0-or-later starting with the v2
licensing transition. The change is not retroactive: all releases and
commits up to v1.7.0 / 74c38a65 (tag mit-final) remain available under MIT.

Since MIT is GPL-compatible, a distribution combining both (such as the
serena-agent package) is as a whole subject to GPL-3.0-or-later, while the
SolidLSP files themselves stay MIT and can be extracted and used separately
under MIT terms. The distribution metadata therefore declares
GPL-3.0-or-later, with both license texts shipped alongside it.

Serena originally began under the GPL (v2) and was switched to MIT in
May 2025 following community requests. We consider that change a mistake;
the substantial changes in v2 make this the appropriate time to revert it.
We want the best version of Serena to remain free.

Changes:
* LICENSE is now the licensing overview; canonical license texts live in
  LICENSES/ (MIT.txt is the previous LICENSE verbatim, GPL-3.0-or-later.txt
  is the unmodified FSF text)
* pyproject.toml declares the PEP 639 license expression
  "GPL-3.0-or-later" and bundles LICENSE and LICENSES/* as license files;
  the deprecated MIT classifier is dropped and flake.nix declares gpl3Plus;
  README has per-component license badges and a License section
* SPDX-License-Identifier headers in all Python sources under src/ and
  scripts/, added by the new idempotent scripts/add_spdx_headers.py, which
  gen_prompt_factory.py also uses to keep the header on the generated
  module; existing third-party notices are preserved
* CLA.md: Contributor License Agreement (contributor retains copyright;
  grants a perpetual, irrevocable license including relicensing under any
  terms, incl. proprietary/commercial; patent grant; authority
  representations), to be enforced repository-wide via cla-assistant.io
* CONTRIBUTING.md, PR template and a new docs page explain the licensing
  boundary and the CLA workflow

CONTRIBUTING.md

@@ -11,6 +11,30 @@ The following types of contributions can be submitted directly via pull requests
 
 For other changes, please open an issue first to discuss your ideas with the maintainers.
 
+## Licensing and Contributor License Agreement (CLA)
+
+Serena is multi-licensed by component (see [LICENSE](LICENSE)):
+the Serena application is licensed under GPL-3.0-or-later, while SolidLSP (`src/solidlsp`) is licensed under MIT.
+
+All contributions to this repository require acceptance of our [Contributor License Agreement](CLA.md).
+The CLA lets you keep the copyright to your work while granting Jain & Panchenko IT-Berater Partnerschaft (Oraios AI) the rights needed to maintain and
+distribute Serena under different terms in the future (including the free community edition and possible
+commercial editions).
+
+Acceptance is handled automatically by [CLA assistant](https://cla-assistant.io/):
+
+* When you open your first pull request, the CLA assistant bot comments on it with a link to accept the CLA.
+* You accept it once, authenticated with your GitHub account; the acceptance is remembered for all future PRs
+  (you will only be asked again if the CLA text changes).
+* PRs cannot be merged until the `license/cla` status check passes.
+
+The CLA is required repository-wide for operational simplicity, i.e. also for SolidLSP-only changes.
+This does not change SolidLSP's MIT license.
+
+When adding new source files, include the SPDX identifier that matches the component:
+`# SPDX-License-Identifier: GPL-3.0-or-later` for Serena application code and
+`# SPDX-License-Identifier: MIT` for SolidLSP.
+
 When submitting a PR, ensure a well-defined scope.
 Every PR should cover a single logical change or a set of closely related changes.