Fix cross-resource s3_copy and AWS config resolution

[PeterSu92]

Discovered during BENCH-9837 work.

The s3_copy tool could not do a recursive cross-resource copy, and every AWS-backed tool could silently use the wrong workspace's credentials.

This PR:

• Copies recursive cross-resource transfers one object at a time. The previous code passed a temp FILE as the destination of aws s3 cp --recursive, which requires a directory, so recursive cross-profile copies always failed. Per-object staging also bounds disk use to the largest single object instead of the whole prefix, which can exceed the staging filesystem.
• Resolves the workspace lazily in ensureAWSConfig via getCurrentWorkspaceUUID(). It previously read cachedWorkspaceUUID, which is only filled at startup, so a workspace set after the server booted left it empty.
• Removes the fallback that returned an arbitrary .conf from ~/.workbench/aws. Those belong to other workspaces, and their profiles resolve to the wrong resources, surfacing as a confusing "config profile could not be found".
• Passes --wb-path when generating the AWS config, so credential_process points at the wb binary in use rather than a path that may not exist.
• Adds WB_MCP_STAGE_DIR to relocate copy staging off the default temp filesystem, which is a small overlay on some images.