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

docs/01-about/060_license.md

@@ -0,0 +1,55 @@
+# License
+
+Serena is **licensed per component**. The authoritative overview is the
+[`LICENSE`](https://github.com/oraios/serena/blob/main/LICENSE) file in the repository,
+and the full license texts are in [`LICENSES/`](https://github.com/oraios/serena/tree/main/LICENSES).
+
+| Component | Location | License |
+|---|---|---|
+| **SolidLSP** – language-server client library, the basis of Serena's free code-intelligence backend | `src/solidlsp`, `test/solidlsp`, `test/resources` | [MIT](https://github.com/oraios/serena/blob/main/LICENSES/MIT.txt) |
+| **Serena application** – everything else (agent, tools, MCP server, prompts, scripts, tests, docs) | `src/serena`, `src/interprompt`, `test/serena`, `scripts`, … | [GPL-3.0-or-later](https://github.com/oraios/serena/blob/main/LICENSES/GPL-3.0-or-later.txt) |
+
+Every source file carries an `SPDX-License-Identifier` line stating which license applies to it.
+
+Because MIT is compatible with the GPL, a distribution that combines both — such as the `serena-agent` package —
+is **as a whole subject to GPL-3.0-or-later**. This does not change the license of the SolidLSP files themselves:
+they remain MIT-licensed and can be extracted and used separately under MIT terms.
+
+## SolidLSP: permissive, reusable infrastructure
+
+SolidLSP is deliberately kept under the permissive MIT license so that it can be reused independently of Serena,
+in any project and under any license. Serena's licensing does not affect SolidLSP.
+
+## Serena: an application that stays free
+
+Starting with the **v2 licensing transition**, the Serena application is licensed under the
+GNU General Public License, version 3 or (at your option) any later version.
+
+### Why the change?
+
+Serena originally began under the GPL. It was later changed to MIT following community requests.
+We have come to consider that change a mistake.
+
+Version 2 introduces substantial architectural and code changes, which made this the appropriate time to reconsider.
+The central principle behind the decision is simple:
+
+> **We want the best version of Serena to remain free.**
+
+The GPL ensures that anyone who redistributes a derivative version of Serena must preserve the same software freedoms
+for its users that they received themselves.
+
+### The change is not retroactive
+
+* All Serena releases and commits up to the historical cutoff documented in `LICENSE` were released under MIT and
+  remain available under MIT.
+* Serena v2 and later application code are GPL-3.0-or-later.
+* Contributions that entered the application under MIT before the cutoff remain incorporated in the GPL-licensed
+  application; the MIT notice continues to apply to that historical code as required by its terms.
+
+## Contributing
+
+Contributions require acceptance of our
+[Contributor License Agreement](https://github.com/oraios/serena/blob/main/CLA.md), which lets you keep your
+copyright while granting the maintainers the rights needed to maintain and relicense Serena (including under
+commercial terms) in the future. Acceptance is a one-time click via the CLA assistant bot on your first pull request.
+See [CONTRIBUTING.md](https://github.com/oraios/serena/blob/main/CONTRIBUTING.md) for details.