Memory env var naming differs between Starter Toolkit and CLI — migration guidance needed

[aidandaly24]

Description

This is not a CLI bug. The CLI has always used dynamic memory env var names (MEMORY_{NAME}_ID). However, the Starter Toolkit uses a different static convention (BEDROCK_AGENTCORE_MEMORY_ID), and users migrating between the two products encounter a silent breakage.

• Starter Toolkit convention: BEDROCK_AGENTCORE_MEMORY_ID (static, single memory)
• CLI convention: MEMORY_{NAME}_ID (dynamic, based on memory resource name, e.g. MEMORY_SHAREDMEMORY_ID)

These are two different products with different env var patterns. They are not directly compatible.

Current State

The agentcore import command (added in #620) already handles this — when importing a Starter Toolkit project, it warns users to update their env var references:

Warning: Memory "SharedMemory" env var must be updated in your agent code:
  - MEMORY_ID = os.getenv("BEDROCK_AGENTCORE_MEMORY_ID")
  + MEMORY_ID = os.getenv("MEMORY_SHAREDMEMORY_ID")

Users migrating from the Starter Toolkit should use agentcore import when possible, which handles the conversion and warns about env var changes.

What's Missing

• Documentation should make it clearer that the CLI and Starter Toolkit use different env var conventions
• Users who don't use agentcore import (e.g. manually recreating their project in the CLI) have no guidance about the env var difference
• Consider adding a note in the memory docs (docs/memory.md) about the naming convention and how it differs from the Starter Toolkit

Context

Reported by a user who was using the Starter Toolkit pattern (BEDROCK_AGENTCORE_MEMORY_ID) and found that the CLI injects MEMORY_SHAREDMEMORY_ID instead. The env var mismatch causes os.getenv() to return None silently at runtime.