chore: relicense to AGPL-3.0-or-later and add SPDX headers

Replaces LICENSE (GPLv3 -> AGPLv3) and prepends
`SPDX-License-Identifier: AGPL-3.0-or-later` to every source file
across decnet/, decnet_web/, tests/, scripts/, and tools/.

Rationale: closes the GPLv3 ASP loophole so any party operating a
modified DECNET as a network service must offer their modified
source. Personal copyright (Samuel Paschuan) + inbound=outbound
contributions make a future unilateral relicense infeasible.

- LICENSE: full AGPL-3.0 text (gnu.org/licenses/agpl-3.0.txt)
- COPYRIGHT: project copyright notice
- tools/add_spdx_headers.py: idempotent header injector
  (shebang- and PEP 263-aware)

Touches 1565 source files (.py, .ts, .tsx, .js, .jsx, .css, .sh).
No behavior change; comments only.

decnet/vectorstore/__init__.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Vector store substrate for behavioral fingerprint similarity search.
 
 Provider-pluggable storage for ``(kind, id, vector)`` triples used by the

decnet/vectorstore/base.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Vector-store abstractions: :class:`BaseVectorStore` ABC + record types.
 
 Every backend (sqlite-vec, in-memory fake, future pgvector / Qdrant)

decnet/vectorstore/factory.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Vectorstore factory — selects a :class:`BaseVectorStore` implementation.
 
 Dispatch keys:

decnet/vectorstore/fake.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """In-memory vector store backend.
 
 Two flavors:

decnet/vectorstore/sqlite_vec.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """SQLite + sqlite-vec backend.
 
 Lazy-imports the ``sqlite_vec`` extension. If the extension isn't