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/agent/__init__.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """DECNET worker agent — runs on every SWARM worker host.
 
 Exposes an mTLS-protected FastAPI service the master's SWARM controller

decnet/agent/app.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Worker-side FastAPI app.
 
 Protected by mTLS at the ASGI/uvicorn transport layer: uvicorn is started

decnet/agent/executor.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Thin adapter between the agent's HTTP endpoints and the existing
 ``decnet.engine.deployer`` code path.
 

decnet/agent/heartbeat.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Agent → master liveness heartbeat loop.
 
 Every ``INTERVAL_S`` seconds the worker posts ``executor.status()`` to

decnet/agent/server.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Worker-agent uvicorn launcher.
 
 Starts ``decnet.agent.app:app`` over HTTPS with mTLS enforcement.  The

decnet/agent/topology_ops.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Agent-side topology apply/teardown/state primitives.
 
 Wraps the compose + bridge machinery from :mod:`decnet.engine.deployer`

decnet/agent/topology_store.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Agent-side sqlite cache of the currently-applied topology.
 
 **This is a cache, not a source of truth.**  The master is the only