Adopt UAPF v2.0.0 cornerstone naming: .bpmn.xml/.dmn.xml/.cmmn.xml -> .bpmn/.dmn/.cmmn [scope-3 3/3]

Renames all 5 cornerstone files to the OMG-ecosystem extensions required
by UAPF-specification v2.0.0, and updates every internal reference:

- uapf.yaml (manifest cornerstone paths + decision refs)
- resources/mappings.yaml (endpoint + usesDecision refs)
- processgit.mcp.yaml (MCP source paths)
- processgit.viewer.json (x2: package + examples)
- cmmn/vertesanas-lieta.cmmn (processTask processRef -> DMN files)
- README.md, docs/source-reference.md, agent.chat.yaml (documentation)

Completes the scope-3 conformance pass for AI-Sandb-Evaluation:
v2.1.0 diagram interchange (CMMN + 3x DMN, prior commits) + v2.0.0 naming.
File contents otherwise unchanged.

processes/L4/vdaa.mi.sandbox.atlase/processgit.viewer.json

@@ -10,9 +10,9 @@
         "examples/candidate-input.example.json"
       ],
       "targets": {
-        "dmn_eligibility": "dmn/01-atbilstiba-un-izsledzosie.dmn.xml",
-        "dmn_scoring": "dmn/02-vertesanas-punkti.dmn.xml",
-        "dmn_trl": "dmn/03-trl-vertejums.dmn.xml",
+        "dmn_eligibility": "dmn/01-atbilstiba-un-izsledzosie.dmn",
+        "dmn_scoring": "dmn/02-vertesanas-punkti.dmn",
+        "dmn_trl": "dmn/03-trl-vertejums.dmn",
         "mappings": "resources/mappings.yaml",
         "guardrails": "guardrails/ai-evaluation-rules.yaml",
         "expected_output": "examples/expected-output.example.json"