feat(core): add trusted execution context contract

[Aias00]

Summary

• add an immutable, backend-neutral TrustedExecutionContext for tenant/principal references, run correlation, deadlines, budgets, and delegation permissions
• keep trusted identity separate from ordinary RunnableConfig metadata and propagate it explicitly through graph, tool, interceptor, and AgentTool child-call paths
• add a strict versioned reference codec plus application resolver so recovery revalidates identity without persisting tokens, credentials, or client objects
• reserve framework ToolContext keys and always replace metadata/interceptor values with the authoritative runtime config

Security invariants

• model arguments and ordinary metadata cannot replace trusted tenant/principal identity
• local delegation can only narrow permissions, deadline, and budget
• concurrent/reactive executions do not use global or ThreadLocal identity state
• live trusted context is transient and excluded from RunnableConfig serialization
• A2A trust propagation and authorization enforcement remain scoped to [P2A][T21] 补齐 A2A 执行关联、稳定消息身份与可信委派上下文传播 #129 and [P0][T03] 在工具执行边界强制校验授权与风险策略 #120

Verification

• ./mvnw -pl :argi-graph-core,:argi-agent-framework test
• make lint
• targeted license-header check for changed Java sources
• independent code review completed with no remaining findings

Closes #119