feat: add documentation for multi-agent coordination test

[rashim27us]

Problem:
Multi_Agent_Coordination.md described distributed kill-switch propagation and worker coordination as safety invariants in prose only. No concrete verification procedures existed for multi-agent failure modes — orchestrator silencing, concurrent scope races, audit trail attribution under halt, inter-agent prompt injection, and heterogeneous autonomy level propagation. Customer_Acceptance_Testing.md covers single-agent kill-switch behavior but has no multi-agent variants, leaving platform builders with nothing testable.

Solution:
Added a Multi-Agent Acceptance Test Scenarios section to Multi_Agent_Coordination.md with 9 structured test procedures (MA-T01–MA-T09). Each test specifies preconditions, numbered steps, exact observable pass/fail signals, evidence to collect, tier applicability, verifier independence requirement, and normative anchors.
Tests cover: concurrent halt propagation, pre-invocation halt check, stale worker detection, rate budget exhaustion, rogue worker isolation, orchestrator silencing with control-plane fallback, audit trail fidelity under halt, concurrent scope boundary races, and heterogeneous autonomy level halt behavior. Updated Getting_Started.md document map accordingly.

Key Achievements:

• MA-T06 (orchestrator silencing) closes the most critical gap: no prior test distinguished halt-via-orchestrator from halt-via-independent-control-plane.
• MA-T05 (rogue worker isolation) extended to cover inter-agent prompt injection, anchored to APTS-MR-002 and APTS-MR-022 — not just out-of-scope action detection.
• All halt tests include a 60-second aggregate state dump SLA assertion across all N workers per APTS-HO-008, not per-worker.
• No new normative requirements introduced — all 9 tests are informative verification procedures against existing requirements, using MA-Txx identifiers and APTS-XX-NNN anchors throughout.