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.

scripts/behave_shell/replay_calibration.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Replay one calibration-corpus shard through the BEHAVE-SHELL handler.
 
 Phase 6 smoke helper. Drives the production handler

scripts/behave_shell/smoke.sh

@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+# SPDX-License-Identifier: AGPL-3.0-or-later
 ### Usage: scripts/behave_shell/smoke.sh [BEHAVE_CALIBRATION_DIR]
 #
 # BEHAVE-INTEGRATION Phase 6 — offline replay smoke test.